Scripting example to provide altitude callouts in feet defaulting to 400ft warnings

[timtuxworth]

Replaces #23368

So many countries have a limit of 400 feet when flying RPAS/UAV (rc planes). This script provides groundstation messages which can be "called out" by QGroundControl, Mission Planner. etc. to advise the pilot what their current altitude is, warn them when approaching 400 feet (or any other set limit) and call out when they have exceeded the limit.

This may not be required on an OpenTX/EdgeTX radio with something like Crossfire/Express LRS as the radio can be configured to do callouts and Yaapu telemetry has a max altitude alert, but EdgeTX won't provide ongoing altitude updates the way this script can do.

After @peterbarker challenged me on this one, I rebased and then had to clean up/improve the example to reflect the code I'm flying now which has changed a bit since the original PR.

Hope it's still ok to merge Peter.

[timtuxworth]

I messed up GitHub somehow @peterbarker so I've created this new PR for the altitude callouts, I also renamed the script and updated it based on the one I'm actually flying.