[generator] Allow 'managedOverride' metadata to support 'abstract' methods.

[jpobst]

Context: dotnet/android#7796

Imagine the following construct, which is new in android.jar API-34:

public abstract class CharBuffer extends Buffer {
  public abstract CharBuffer duplicate ();
}

public abstract class Buffer  {
  public abstract Buffer duplicate ();
}

Generating the subclass's abstract method causes:

error CS0533: 'CharBuffer.Duplicate()' hides inherited abstract member 'Buffer.Duplicate()'

C# supports this construct if we make the subclass's abstract method with override:

public abstract class CharBuffer : Buffer {
  public override abstract CharBuffer Duplicate ();
}

Theoretically we can tell generator how to fix this thanks to our addition of the managedOverride metadata property:

<attr path="/api/package[@name='java.nio']/class[@name='CharBuffer']/method[@name='duplicate']" name="managedOverride">override</attr>

However, managedOverride was not written to support abstract methods. This commit fixes that issue so that using managedOverride to create an override abstract method is possible.

Method Invokers

This gets us halfway there, but there is another issue: method invokers for both the class and base class methods will be generated:

[global::Android.Runtime.Register ("java/nio/CharBuffer", DoNotGenerateAcw=true)]
internal partial class CharBufferInvoker : CharBuffer {
	[Register ("duplicate", "()Ljava/nio/CharBuffer;", "")]
	public override sealed unsafe CharBuffer Duplicate () { ... }

	[Register ("duplicate", "()Ljava/nio/Buffer;", "")]
	public override sealed unsafe Buffer Duplicate () { ... }
}

This creates the following error:

error CS0111: Type 'CharBufferInvoker' already defines a member called 'Duplicate' with the same parameter types
error CS0508: 'CharBufferInvoker.Duplicate()': return type must be 'CharBuffer' to match overridden member 'CharBuffer.Duplicate()'

This perhaps(?) could be something generator could detect and prevent, but the feasibility and cost is unknown and expected to be high. Since this is a very rare case, we will fix it with metadata, however we have never had metadata to apply to invokers before.

We have decided to add a skipMethodInvokers metadata to the class that the invoker class would be created for:

<attr path="/api/package[@name='java.nio']/class[@name='CharBuffer']" name="skipInvokerMethods">java/nio/Buffer.duplicate()Ljava/nio/Buffer;</attr>

This tells generator to skip generating the Buffer Buffer.duplicate () invoker when generating the CharBuffer type. Multiple invokers to skip can be specified as a comma or space separated list.

[jonpryor]

I think space could make fore a "more readable" representation.

[jonpryor]

This should use StringSplitOptions.RemoveEmptyEntries: https://learn.microsoft.com/en-us/dotnet/api/system.string.split?view=net-7.0#system-string-split(system-char()-system-stringsplitoptions)

[jonpryor]

Should also add \n and \r in there as well.