GroupedNativeMethodsGenerator: generate attributes on extension methods where applicable

[Xpl0itR]

Adds the following attributes where applicable:

• MethodImplAttribute with AggressiveInlining hint to generated extension methods
• GeneratedCodeAttribute to generated extensions class
• ObsoleteAttribute to generated extension methods where the native method binding is also marked Obsolete

[neuecc]

Thank you.
GeneratedCodeAttribute is for CodeDom and is an old one, isn't it?
For things like AggressiveInlining that are generated, the JIT should perform it automatically, so I think it's not necessary to explicitly specify it.

[Xpl0itR]

GeneratedCodeAttribute is useful for tools that operate on the assembly rather than the source file because they won't see the // <auto-generated/> header. It is still used by Microsoft in their source generators, like System.Text.Json for example.

Similarly, Microsoft explicitly use AggressiveInlining in the standard libraries like the Unsafe.AsPointer for example.