Support `editor_register_get_classes_used_callback` with `api-custom-json` feature

https://github.com/godotengine/godot/pull/104129 added `editor_register_get_classes_used_callback` which can be used to limit classes included in particular project – we should provide a list of used classes in case if `api-custom-json` feature is used.

Without `api-custom-json` || `lazy-function-tables` all the classes are required anyway – thus supporting this callback in other cases makes no sense.

As of today figuring out what classes are being used by given project is not being done by godot-rust and we shouldn't provide this functionality; It is user responsibility to provide proper `extension_api.json` from which used classes can be derived (in the future someone might create command line tool or plugin to support that).