Allow GDExtensions to register virtual methods and call them on scripts (godot-cpp support)

[dsnopek]

This depends on PR godotengine/godot#87758

It adds APIs compatible with Godot's, including the following macros:

• GDVIRTUAL*() (like GDVIRTUAL0R(), GDVIRTUAL2C(), etc)
• GDVIRTUAL_BIND()
• GDVIRTUAL_CALL()
• GDVIRTUAL_IS_OVERRIDDEN()
• ... and all the rest!

It also adds an automated test for this functionality to CI.

NOTE: The tests here will fail until after the Godot PR is merged! However, they are passing for me locally.

Fixes #1199
Fixes #1072
Fixes #910

[Remi123]

I've been waiting for this feature for over a year. Thank you so much.

Glad to see an example. 4.3 will be great for plugin.

[akien-mga]

Didn't review in depth, but looks good at a glance. We merged the upstream counterpart so this could be merged now.

[Kehom]

I finally got back to the code I needed this feature. I have noticed that the macros defined in the gdvirtual.gen.inc file begin with StringName _gdvritual_ without explicit namespace usage. This somewhat forces extension code to add using namespace godot.

Is this intended?

EDIT: I attempted to create a custom macro copying the original one but adding the explicit godot::NameString thing. It didn't work. Nevertheless, the macros do work if I add the using namespace godot (which is not exactly something I wanted to do).

[dsnopek]

Is this intended?

Nope! This is probably just an artifact of those macros starting out as a copy of the same macros from Godot.

[dsnopek]

@Kehom Can you test if this PR fixes it for you? #1412 Unfortunately, I don't have a project that doesn't use using namespace godot at hand to test if those changes are sufficient

[Kehom]

Thanks for the answer. I think it will be best for me to answer about the fix in the PR itself.