GCS_Mavlink: enable mavlink2 signing on USB port

As described on our [security wiki page](https://ardupilot.org/dev/docs/security-landing-page.html) we support various methods of making a vehicle more resistant to malicious actors but one key requirement is, "The first step in protecting the vehicle is to ensure that it is physically secure".

While we can't protect against vandalism we could better protect data stored on the autopilot (e.g. parameters, log files, mission commands, fences, rally points, lua scripts) from being downloaded if we implemented mavlink2 signing on the USB port. This would not protect against this data being retrieved directly from the SD card but that could be addressed separately.

### Platform

- [x] All
- [ ] AntennaTracker
- [ ] Copter
- [ ] Plane
- [ ] Rover
- [ ] Submarine

Read the linked security wiki page first, then trace the GCS_Mavlink handling for the USB port and its MAVLink2 signing path. Determine the required signing setup and security behavior before identifying suitable tests; done means USB communications protect the listed autopilot data as described without weakening other platform connections.

Written by the indexing model from the issue text.