GDBusAuthObserver: Add a way to control what authentication mechanisms to use

This is related to https://bugzilla.gnome.org/show_bug.cgi?id=673943
but also useful in a lot of other contexts.

gio/gdbusconnection.c

@@ -2553,6 +2553,7 @@ initable_init (GInitable     *initable,
       g_assert (connection->guid == NULL);
       connection->auth = _g_dbus_auth_new (connection->stream);
       connection->guid = _g_dbus_auth_run_client (connection->auth,
+                                                  connection->authentication_observer,
                                                   get_offered_capabilities_max (connection),
                                                   &connection->capabilities,
                                                   cancellable,