Allow objects to be added/removed from method calls

refi64[m]12:

Hey so I'm trying to use zbus for a personal project (on the main branch for that sweet, sweet async server support), but I think I hit a bit of a minor roadblock? It seems there's no way to get a mutable ObjectServer inside of a handler (using #[zbus(object_server)] gives me a &ObjectServer), meaning I can't export a new object from within a handler. In my case, the method is supposed to return the new object handle, so offloading that via tokio::spawn and calling object_server_mut from there or something isn't really an option. Would there be any chance of adding a #[zbus(object_server_mut)] option or similar? (I could send an MR for it if needed.)

I think the easiest solution is to make ObjectServer::at a &self method.