Implement callable_mp() and callable_mp_static()

[dsnopek]

This is the godot-cpp counterpart to PR godotengine/godot#79005 (which this PR depends on).

It very minimally works! It adds a single test case that passes :-)

However, it's a draft because:

1. I want to add more test cases to ensure it works with void functions and const methods that return a value
2. I haven't added callable_mp_static() yet

This is now ready for review and has full test coverage!

Unfortunately, the tests will fail on GitHub CI until the Godot PR is merged.

Fixes #1089

[dsnopek]

callable_mp_static() is implemented in my latest push, and there's now full test coverage. So, this is ready for review!

Unfortunately, the tests will fail on GitHub CI until the Godot PR is merged.

[rburing]

This looks great! This will remove the need for a (registered) dummy "factory" class/method to register a physics server from a GDExtension, and allow using a static callback instead, as it's done in core.

cc @mihe for Godot-Jolt.

[ozzr]

I have been waiting for this.
The engine has way too many useful macros that should be exposed to GDExtension

[dsnopek]

I just updated this to work with @maiself's alternative PR godotengine/godot#79005

[akien-mga]

The upstream PR was merged, so this can be revisited.

[dsnopek]

I've just rebased this on the latest master. It should finally pass tests (which include some unit tests actually using callable_mp() and callable_mp_static()).

As discussed on this comment, I would like to expand what godot-cpp does with the new custom callable API that was added in Godot:

so the Godot PR adds the API to gdextension_interface.h to create custom callables, but the godot-cpp PR only uses that API to implement callable_mp() and callable_mp_static(), it doesn't provide a base class for making your own custom callables. But that's a good idea! When I have a chance I think I'll try to extend the godot-cpp PR to include something like that.

But I think that can be saved for a follow-up PR. What's added here is already super useful, and it doesn't get in the way of expanding the functionality later.

[BastiaanOlij]

Reviewed during GDExtension meeting, this is looking really good!