Callable: add callv method

[rburing]

Analogous to the existing callv method on Object, this new method on Callable takes a single array of arguments.

This method has the advantage that it can be called from GDExtension without vararg template shenanigans. For example, the automatically generated bindings in godot-cpp will work out of the box. This is important as it provides the first way to actually call a Callable using godot-cpp. (Better ways may be implemented later.)

Together with #65828, this will unblock ongoing work on custom physics servers.

[bruvzg]

Looks good to me, since Object have one, I do not see any reason not to have it in Callable.

[akien-mga]

Thanks!