Added `ClassDB::bind_static_vararg_method()` to bind static variadic functions.

[ttalexander2]

I needed to bind static variadic functions for a GDExtension project i'm working on, so I figured I'd create a PR.

Adds ClassDB::bind_static_vararg_method(). I tried by best to mimic the conventions in which member vararg methods are bound.

[dsnopek]

Thanks, this is an interesting feature!

However, one of the design goals of godot-cpp is to provide (as much as possible) the same API as used internally by Godot modules. Since Godot's ClassDB doesn't support this function, it'll need to be added there first.