[generator] Emit partial method for peer constructor

Context: #1085
Context: dotnet/runtime#82121

Some Java objects are *big*, e.g. [`Bitmap`][0] instances, but as
far as MonoVM is concerned, the `Bitmap` instances are *tiny*: a
few pointers, and that's it.  MonoVM doesn't know that it could be
referencing several MB of data in the Java VM.

MonoVM is gaining support for [`GC.AddMemoryPressure()`][1] and
[`GC.RemoveMemoryPressure()`][2], which potentially allows for the
parent of all cross-VM GC integrations: using the `GC` methods to
inform MonoVM of how much non-managed memory has been allocated.
This could allow MonoVM to collect more frequently when total process
memory is low.

How should we call `GC.AddMemoryPressure()` and
`GC.RemoveMemoryPressure()`?

`GC.RemoveMemoryPressure()` is straightforward: a class can override
`Java.Lang.Object.Dispose(bool)`.

`GC.AddMemoryPressure()` is the problem: where and how can it be
called from a class binding?  This is trickier, as there was no way
to have custom code called by the bound type.  Instead, various
forms of "hacky workarounds" are often employed, e.g. copying
`generator`-emitted content into a `partial` class, then using
`metadata` to *prevent* `generator` from binding those members.
It's all around fugly.

Fortunately C# has a solution: [`partial` methods][3]!

We take our existing "peer constructor" generation code, a'la:

	partial class Bitmap : Java.Lang.Object {
	    internal Bitmap (IntPtr h, JniHandleOwnership t) : base (h, t)
	    {
	    }
	}

and extend it to:

	partial class Bitmap : Java.Lang.Object {
	    partial void _OnBitmapCreated ();
	    internal Bitmap (IntPtr h, JniHandleOwnership t) : base (h, t)
	    {
	        _OnBitmapCreated ();
	    }
	}

This allows a `partial class Bitmap` to do:

	// Hand-written code
	partial class Bitmap {
	    int _memoryPressure;

	    partial void _OnBitmapCreated ()
	    {
	        _memoryPressure = ByteCount;
	        GC.AddMemoryPressure (_memoryPressure);
	    }

	    protected override void Dispose (bool disposing)
	    {
	        if (_memoryPressure != 0) {
	            GC.RemoveMemoryPressure (_memoryPressure);
	            _memoryPressure = 0;
		}
	    }
	}

TODO: Are there any other places where such `partial` methods would
be useful?  This appears to be the minimum required for this scenario.

[0]: https://developer.android.com/reference/android/graphics/Bitmap
[1]: https://learn.microsoft.com/dotnet/api/system.gc.addmemorypressure?view=net-7.0
[2]: https://learn.microsoft.com/dotnet/api/system.gc.removememorypressure?view=net-7.0
[3]: https://learn.microsoft.com/dotnet/csharp/language-reference/keywords/partial-method

Author: jonpryor

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteClass.txt

@@ -3,8 +3,12 @@
 public partial class MyClass {
 	static readonly JniPeerMembers _members = new JniPeerMembers ("java/code/MyClass", typeof (MyClass));
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (ref JniObjectReference reference, JniObjectReferenceOptions options) : base (ref reference, options)
 	{
+		_OnMyClassCreated ();
 	}
 
 	// Metadata.xml XPath constructor reference: path="/api/package[@name='java.code']/class[@name='MyClass']/constructor[@name='MyClass' and count(parameter)=0]"

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteKotlinUnsignedArrayTypeMethodsClass.txt

@@ -3,8 +3,12 @@
 public partial class MyClass {
 	static readonly JniPeerMembers _members = new JniPeerMembers ("java/code/MyClass", typeof (MyClass));
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (ref JniObjectReference reference, JniObjectReferenceOptions options) : base (ref reference, options)
 	{
+		_OnMyClassCreated ();
 	}
 
 	// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/class[@name='MyClass']/method[@name='Echo' and count(parameter)=1 and parameter[1][@type='uint[]']]"

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteKotlinUnsignedArrayTypePropertiesClass.txt

@@ -3,8 +3,12 @@
 public partial class MyClass {
 	static readonly JniPeerMembers _members = new JniPeerMembers ("java/code/MyClass", typeof (MyClass));
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (ref JniObjectReference reference, JniObjectReferenceOptions options) : base (ref reference, options)
 	{
+		_OnMyClassCreated ();
 	}
 
 	public unsafe Java.Interop.JavaInt32Array? UIntProp {

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteKotlinUnsignedTypeMethodsClass.txt

@@ -3,8 +3,12 @@
 public partial class MyClass {
 	static readonly JniPeerMembers _members = new JniPeerMembers ("java/code/MyClass", typeof (MyClass));
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (ref JniObjectReference reference, JniObjectReferenceOptions options) : base (ref reference, options)
 	{
+		_OnMyClassCreated ();
 	}
 
 	// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/class[@name='MyClass']/method[@name='Echo' and count(parameter)=1 and parameter[1][@type='uint']]"

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteKotlinUnsignedTypePropertiesClass.txt

@@ -3,8 +3,12 @@
 public partial class MyClass {
 	static readonly JniPeerMembers _members = new JniPeerMembers ("java/code/MyClass", typeof (MyClass));
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (ref JniObjectReference reference, JniObjectReferenceOptions options) : base (ref reference, options)
 	{
+		_OnMyClassCreated ();
 	}
 
 	public unsafe uint UIntProp {

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteMethodWithCharSequenceArrays.txt

@@ -9,8 +9,12 @@ public partial class MyClass : Java.Lang.Object {
 		get { return _members; }
 	}
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (ref JniObjectReference reference, JniObjectReferenceOptions options) : base (ref reference, options)
 	{
+		_OnMyClassCreated ();
 	}
 
 	// Metadata.xml XPath method reference: path="/api/package[@name='com.example']/class[@name='MyClass']/method[@name='echo' and count(parameter)=1 and parameter[1][@type='java.lang.CharSequence[]']]"

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteObjectField.txt

@@ -23,8 +23,12 @@ public partial class MyClass {
 
 	static readonly JniPeerMembers _members = new JniPeerMembers ("java/code/MyClass", typeof (MyClass));
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (ref JniObjectReference reference, JniObjectReferenceOptions options) : base (ref reference, options)
 	{
+		_OnMyClassCreated ();
 	}
 
 }

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1-NRT/WriteClass.txt

@@ -7,8 +7,12 @@ public partial class MyClass {
 		get { return _members.JniPeerType.PeerReference.Handle; }
 	}
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (IntPtr javaReference, JniHandleOwnership transfer) : base (javaReference, transfer)
 	{
+		_OnMyClassCreated ();
 	}
 
 	// Metadata.xml XPath constructor reference: path="/api/package[@name='java.code']/class[@name='MyClass']/constructor[@name='MyClass' and count(parameter)=0]"

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1-NRT/WriteMethodWithCharSequenceArrays.txt

@@ -25,8 +25,12 @@ public partial class MyClass : Java.Lang.Object {
 		get { return _members.ManagedPeerType; }
 	}
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (IntPtr javaReference, JniHandleOwnership transfer) : base (javaReference, transfer)
 	{
+		_OnMyClassCreated ();
 	}
 
 	static Delegate? cb_echo_arrayLjava_lang_CharSequence_;

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1-NRT/WriteObjectField.txt

@@ -29,8 +29,12 @@ public partial class MyClass {
 		get { return _members.JniPeerType.PeerReference.Handle; }
 	}
 
+	/// <summary>Extension point; called when creating a Managed Peer around a preexisting Java Peer instance.</summary>
+	partial void _OnMyClassCreated ();
+
 	protected MyClass (IntPtr javaReference, JniHandleOwnership transfer) : base (javaReference, transfer)
 	{
+		_OnMyClassCreated ();
 	}
 
 }