`GeometryInstance::set_custom_aabb()` is not exported to cpp bindings

[Flarkk]

The method GeometryInstance::set_custom_aabb(...) is not available in cpp bindings.

See method documentation here : https://docs.godotengine.org/fr/stable/classes/class_geometryinstance.html#class-geometryinstance-method-set-custom-aabb

The workaround is to directly call VisualServer::get_singleton()->instance_set_custom_aabb(...);