add mqtt message

CMakeLists.txt

@@ -82,6 +82,7 @@ add_message_files(
         Waypoint.msg
         StampedVelocity.msg
         StampedInZone.msg
+        MqttMessage.msg
 )
 
 ## Generate services in the 'srv' folder

msg/MqttMessage.msg

+# an MQTT topic that is not treated by a special handler
+string topic                 # the mqtt topic
+int8[] content               # the content of the message as bytes (int8 in the msg format)
\ No newline at end of file