[MP] Add a "synchronized" signal to MultiplayerSynchronized.

Emitted upon receiving a valid sync packet after setting the variables
state.

modules/multiplayer/scene_replication_interface.cpp

@@ -775,6 +775,7 @@ Error SceneReplicationInterface::on_sync_receive(int p_from, const uint8_t *p_bu
 		err = MultiplayerSynchronizer::set_state(props, node, vars);
 		ERR_FAIL_COND_V(err, err);
 		ofs += size;
+		sync->emit_signal(SNAME("synchronized"));
 #ifdef DEBUG_ENABLED
 		_profile_node_data("sync_in", sync->get_instance_id(), size);
 #endif