Adds VideoStream and relevant resource loaders to migrate
external GDNative plugins to GDExtension.
Adds a VideoStreamLoader as a specialization of ResourceFormatLoader
as ClassDB::is_parent_class is inaccessible from GDExtension currently.
Using Object* instead of Ref<T> in order to avoid the refcount bug
(godotengine/godot-cpp#652)
Also another bug is in ResourceLoader in use on the extension side that
requires fixing.
