Message Signing - updates on implementations

[hamishwillee]

Message Signing · MAVLink Developer Guide

Mission planner/ArduPilot have an implementation. This should be linked. The discussion on this can be found here:

• ArduPilot/ardupilot_wiki#1553
• https://discuss.ardupilot.org/t/enabling-mavlink-2-0-and-packet-signing/25656/11
• Mavproxy docs: https://ardupilot.github.io/MAVProxy/html/modules/signing.html