Add in a special serial message to get the list of serial -> topic mappings

[clalancette]

The way the current serial protocol works is that an 8-bit quantity represents the "topic_ID", which is really a representation of the (topic_type, topic_name) tuple. The map of topic_ID -> (topic_type, topic_name) is currently given in a YAML file that is passed as ROS 2 parameters at ros2_to_serial_bridge startup time. It would be nice to allow this information to come either from the YAML file, or to be queried directly from the far end of the serial connection. That way the microcontroller on the far end could be completely self-describing and ros2_to_serial_bridge could be completely generic.