Revisit godot-cpp interfaces bindings

Some of the server wrapper classes (e.g: `ARVRServer`) provide the ability to register subclasses to be invoked at runtime (e.g: `ARVRService::add_interface(...)`) by the core engine.

However since the provided subclasses are actually instances of the godot-cpp wrapper instead of the target class, these APIs are no-op as there is no logic to forward the calls from the target class to the wrapper class.