Support builtin class methods with varcall

[Bromeon]

The following methods on builtins take varargs and thus follow the varcall convention:

Callable::call
Callable::call_deferred
Callable::rpc
Callable::rpc_id
Callable::bind
Signal::emit

The variant_get_ptr_builtin_method API returns null when invoked for the respective variant types, methods names and hashes.

This likely needs an additional API on GDExtension side.

[Bromeon]

Related:

• Set vararg methods' ptrcall of builtin classes, and let them can be called without arguments. godotengine/godot#76047
• Implement vararg methods of builtin classes. godotengine/godot-cpp#1091

[Bromeon]

Looks like Godot implemented support for builtin methods via varcall, so we could provide them as well.

[TitanNano]

This is available from 4.2.

[ansdor]

I just started using godot-rs and I'm loving it so far, but Callable::bind is a very significant missing piece. Binding arguments to callables is the most convenient way to do some things in godot, such as when you have a list of buttons and you want to connect their "pressed" signals to the same receiving function and you want the buttons to identify themselves to the receiver

I wish I could do this myself but I'm not nearly familiarized enough with godot or godot-rs to write the code. But if there's no more objections on the godot C++ side I'd really love to see this implemented on godot-rs