C#: Implement readonly-ness in Array and Dictionary

[raulsntos]

• Implement support for setting Arrays as readonly
  • Added to Core in Implement read-only arrays #61127.
  • Expose IsReadOnly property that checks the read_only ptr in the engine array.
  • Add MakeReadOnly method.
    • There's precedent in System.Text.Json for a method MakeReadOnly that makes the JsonSerializerOptions readonly, there's no way to make it writable again.
  • Calls that mutate the Array when it's read-only will throw an InvalidOperationException like other read-only collections in .NET.
• Implement support for setting Dictionaries as readonly
  • Added to Core in Implement read-only dictionaries. #61087.
  • Expose IsReadOnly property that checks the read_only ptr in the engine dictionary.
  • Add MakeReadOnly method.
    • Just like with Array, it makes the collection immutable but cannot make it mutable again after that.
  • Calls that mutate the Dictionary when it's read-only will throw an InvalidOperationException like other read-only collections in .NET.

[raulsntos]

This is pending changes to Core that may requisite changing the implementation of this PR (see #71845 (comment)).

Edit: Changes made, following the merge of #71845.

[akien-mga]

Thanks!