Guard against (de)serializing pointers & ref structs

[layomia]

Fixes #34644.

Ref structs are explicitly not supported for deserialization following this PR; created an issue to consider support in the future - #34779. Prior to this change we leak a TypeLoadException:

 typeof(System.TypeLoadException): The generic type 'System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1' was used with an invalid instantiation in assembly 'System.Text.Json, Version=5.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
        ---- System.TypeLoadException : The generic type 'System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1' was used with an invalid instantiation in assembly 'System.Text.Json, Version=5.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
        Stack Trace:
             at System.RuntimeTypeHandle.Instantiate(QCallTypeHandle handle, IntPtr* pInst, Int32 numGenericArgs, ObjectHandleOnStack type)
          D:\repos\dotnet_runtime_2\src\coreclr\src\System.Private.CoreLib\src\System\RuntimeHandles.cs(486,0): at System.RuntimeTypeHandle.Instantiate(Type[] inst)
          D:\repos\dotnet_runtime_2\src\coreclr\src\System.Private.CoreLib\src\System\RuntimeType.CoreCLR.cs(3226,0): at System.RuntimeType.MakeGenericType(Type[] instantiation)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\Converters\Object\ObjectConverterFactory.cs(38,0): at System.Text.Json.Serialization.Converters.ObjectConverterFactory.CreateConverter(Type typeToConvert, JsonSerializerOptions options)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonConverterFactory.cs(60,0): at System.Text.Json.Serialization.JsonConverterFactory.GetConverterInternal(Type typeToConvert, JsonSerializerOptions options)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializerOptions.Converters.cs(192,0): at System.Text.Json.JsonSerializerOptions.GetConverter(Type typeToConvert)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializerOptions.Converters.cs(105,0): at System.Text.Json.JsonSerializerOptions.DetermineConverter(Type parentClassType, Type runtimePropertyType, PropertyInfo propertyInfo)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonClassInfo.cs(343,0): at System.Text.Json.JsonClassInfo.GetConverter(Type type, Type parentClassType, PropertyInfo propertyInfo, Type& runtimeType, JsonSerializerOptions options)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonClassInfo.Cache.cs(88,0): at System.Text.Json.JsonClassInfo.AddProperty(Type propertyType, PropertyInfo propertyInfo, Type parentClassType, JsonSerializerOptions options)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonClassInfo.cs(119,0): at System.Text.Json.JsonClassInfo..ctor(Type type, JsonSerializerOptions options)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializerOptions.cs(340,0): at System.Text.Json.JsonSerializerOptions.GetOrAddClass(Type type)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\WriteStack.cs(71,0): at System.Text.Json.WriteStack.Initialize(Type type, JsonSerializerOptions options, Boolean supportContinuation)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializer.Write.Helpers.cs(23,0): at System.Text.Json.JsonSerializer.WriteCore[TValue](Utf8JsonWriter writer, TValue value, Type inputType, JsonSerializerOptions options)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializer.Write.ByteArray.cs(67,0): at System.Text.Json.JsonSerializer.WriteCoreBytes[TValue](TValue value, Type inputType, JsonSerializerOptions options)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializer.Write.ByteArray.cs(53,0): at System.Text.Json.JsonSerializer.SerializeToUtf8Bytes(Object value, Type inputType, JsonSerializerOptions options)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\tests\Serialization\SerializationWrapper.cs(32,0): at System.Text.Json.Serialization.Tests.SerializationWrapper.SpanSerializerWrapper.Serialize(Object value, Type inputType, JsonSerializerOptions options)
          D:\repos\dotnet_runtime_2\src\libraries\System.Text.Json\tests\Serialization\InvalidTypeTests.cs(67,0): at System.Text.Json.Serialization.Tests.InvalidTypeTests.<>c__DisplayClass5_0.<TypeWithInvalidMember>b__1()

[ahsonkhan]

Ref structs are explicitly not supported for deserialization following this PR; created an issue to consider support in the future - #34777.

I think you meant #34779.

[layomia]

Test failure unrelated.