Add _nm_object_class_add_interface(), for declaring that a class
implements a particular interface, and then have NMObject create the
corresponding proxies itself. (The subclass can get a copy with
_nm_object_get_proxy() if it needs it for something).

(In GDBus, creating a proxy is a heavier operation than in dbus-glib,
so we'll need to create the proxies asynchronously. Moving the
creation to NMObject makes that easier since we can do it as part
of the existing init/init_async.)