AP_DDS: Support throttle suppress in GUIDED mode for plane

[Ryanf55]

Feature request

Is your feature request related to a problem? Please describe.

I can't use GUIDED mode to turn off the throttle on a motor glider using either mavlink or DDS.

Describe the solution you'd like

Guided should respect overrides so you can suppress throttle.

Describe alternatives you've considered

Adding offboard mode

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Additional context

I want to ensure the motor is off, commanding target speed does not do that.
#4337
This previous issue says that disabling overrides in guided was by design, but this is limiting guided to a subset of the control that other modes have.

See SoaringController::suppress_throttle which requires a reference to the TECS controller. Maybe guided can get a reference to TECS too? The tecs controller is private to the Plane object.

Future enhancement

Give DDS full throttle control in GUIDED

[peterbarker]

The referenced issue #4337 doesn't have anything to do with servo outputs. Overriding your throttle to be zero won't make an inch of difference.

I'm not sure this is something that should go into "guided". Perhaps into that new "offboard" mode I've been hearing about?

[Ryanf55]

Approach discussed today: set_gliding_state ROS -> AP_DDS-> AP_ExternalControl -> AP_ExternalControl_Plane -> plane.TECS_controller.set_gliding_requested_flag

See PX4/PX4-Autopilot#14643 and https://github.com/Jaeyoung-Lim/mav-soaring/tree/master for how POSITION_TARGET_LOCAL_NED was used to suppress throttle.

Also mentioned - we should remove the gliding requested flag on mode exit so you don't get stuck without motor if the plane failsafes.