[NET_HANDLER] Work with Shepherd to implement hearbeat

[benliao1]

Shepherd currently has some trouble detecting robot disconnects. They rely on Runtime cleanly exiting in the event of an emergency of some sort, and closing the server-side port. However, in some of these edge cases, Runtime isn't really able to close that socket well (ex. networking error / disconnect, robot shutdown because of low voltage, etc.). It would be easier to simply implement a heartbeat that is sent from Runtime to Shepherd periodically (maybe once per second or so) which would let Shepherd know that the robot is still alive and well.

[benliao1]

This could probably be implemented as a TEXT protobuf that is sent with a timestamp or is blank or something, just a simple message saying "I'm alive!"

[benliao1]

This will be implemented with the Runtime Status Message feature.