dbus_interface methods don't have access to request MessageHeaders

Right now integrating zbus_polkit with zbus means that users have to implement the Interface trait mnaually, which means a lot of boilerplate just to support polkit and is generally a lot of work. I think it'd be nice if polkit support was baked into the dbus_interface macro or if zbus_polkit provided a separate macro for this, so users don't need to write as much boilerplate and potentially introduce bugs, which could be rather fatal with polkit code (e.g. due to accidentally allowing access for actually unauthorized parties)