How should protected Mavlink endpoints behave?

Today, endpoints in companion have a Boolean field called "protected".
When protected, those endpoints cannot be removed by the end-user, via REST API. One can only remove this endpoint by directly calling the ArduPilotManager's delete method.

As the end-user cannot remove protected endpoints, it cannot create them also (or no one would ever remove it, with currently implementation).

@patrickelectric commented he doesn't think this should be the final behaviour of this API.

This issue was opened so we can discuss what is the final desired behavior, and modify it accordingly.

DAYS_TO_DEVELOP:1