Add support for MAV_CMD_DO_SET_ROI_WPNEXT_OFFSET

[peterbarker]

Closes #7658

I've only tested this as far as QGC can upload a site-scan mission and the vehicle seems to run it through to completion.

I have not made sure the gimbal actually tracks the next waypoint location properly...

Now includes a simple autotest.

The the way this waypoint works seems to be pretty straight forward in its use by sitescan. The vehicle flies parallel to a polygon defining the structure. The gimbal is instructed to point towards the next waypoint - but with a 90-degree yaw - so pointing at the structure. In the case of a circular structure the craft follows a regular polygon approximating a circle - so I would imagine that the gimbal would not keep the circular building centered in the frame.

I think this functionality could probably reasonably be put behind a define....

[rmackay9]

nice to see all these gimbal related enhancements! I'm not sure how much use the DO_SET_ROI_WPNEXT_OFFSET command will get but I added that issue because Don from QGC dev said there were some mapping related features that wouldn't work in AP without these.

[rmackay9]

This looks pretty good to me but it seems like maybe only the Gremsy and Servo gimbals are supported? We should add support for all gimbals really.

I'm slightly surprised (but not really) that we need a new gimbal mode to do this but I think this is essentially an existing issue that will only be fixed by moving the mode handling up into the backend code... that's beyond the scope of this PR though so if we can at least get all the other backends to support this new mode then I'm happy.

[rmackay9]

There's a request for this feature here in the forums. https://discuss.ardupilot.org/t/viewpro-gimbal-point-camera-here-function/98279/27