Expose callable_mp macros in godot-cpp?

[RonYanDaik]

Engine has a macros callable_mp which uses class CallableCustomMethodPointerRetC that allow to assign native method. But I cant find analogue in GDExtension.

Godot 4.0

[dsnopek]

This is added by this pair of PRs:

• Allow CallableCustom objects to be created from GDExtensions godot#78813
• Implement callable_mp() and callable_mp_static() #1155