Copter: support per motor compassmot

Copter-3.6 supports per-motor compass mot so we should probably allow the option of kicking this off from the mission planner. there is a [discussion here](https://discuss.ardupilot.org/t/implementing-per-motor-calibration) including the mavlink command that should be sent to kick it off.

Start with the linked discussion on implementing per-motor calibration and confirm the MAVLink command described there. Then locate Mission Planner’s existing compassmot or calibration entry point and determine how the per-motor option should be exposed; done means the calibration can be initiated from Mission Planner using the correct command.

Written by the indexing model from the issue text.