Rover: support do-set-yaw on vehicles that support lateral control

For rovers and boats with lateral control, it should be possible to point the vehicle in any direction (i.e. pointing at a point of interest) but still have the vehicle proceed towards the target location/waypoint in Auto or Guided. The steps to add support include the following:

- Add a lateral speed controller to APM_Control/AR_AttitudeControl. This should probably be quite similar to the existing speed controller.
- Enhance AR_WPNav so that the output L1 controller are converted to output throttle and lateral output instead of throttle-and-steering
- Back in the vehicle code (mode.cpp probably?) retrieve the lateral output from the AR_WPNav and pass it to the motors library

This request comes from [this discussion](https://discuss.ardupilot.org/t/support-for-omni-boats-coming-in-rover-3-5/31663/19).

Start by reading the existing speed controller in APM_Control/AR_AttitudeControl, then trace AR_WPNav outputs into mode.cpp and the motors library. The work is complete when vehicles with lateral control can use do-set-yaw while travelling toward an Auto or Guided target, with throttle and lateral outputs passed through the relevant controllers.

Written by the indexing model from the issue text.