Add topic remapping to launch

* Add topic remapping to ease the setup of controlling gimbal from
  an external drone.
* Remap /external/ros2_gremsy/gimbal_goal to the drone's own namespace

launch/ros_gremsy_gimbal.launch.py

@@ -35,6 +35,9 @@ def generate_launch_description():
 
             }],
             output='screen',
+            remappings=[
+                ('ros2_gremsy/gimbal_goal', '/external/ros2_gremsy/gimbal_goal'),
+            ],
         ),
     ),