[Feature]: Add messages to set/get the navball speed display mode #57

The FlightGlobals API defines a Enum related to the speed display mode of the navball (orbit, surface or target) and some functions to get/set it. The idea is to use this API to have a message that set the mode to a given value and another to sent the current mode to the Arduino.

Those two messages will be simple (a single byte each). The message from KSP to Arduino can be only sent when there is a change (like the AG messages).

This is a good first issue as there is a lot of example of how to handle both inbound and outbound messages and the KSP API seems straightforward.