Copter: Support specifying yaw as well as gimbal rate in GUIDED mode scenarios

#### Issue details

Yet another call for being able to programmatically set the rate of yaw and rate of changing gimbal attitude (particularly pitch) to allow for obtaining smooth video in a dynamic scene when the subject is in motion and where that motion can only be approximately predicted (i.e future motion is not pre-planned).
Currently, copter motion can be re-planned smoothly using set velocity, but the same is not true for yaw and gimbal attitude.

This does seem to be supported already assuming the RC input method and can be approximated using ROI, so it appears possible in theory at least from a control systems standpoint, so this might only be a matter of offering up a more flexible API. The problem with the current system, though, is that it assumes either an RC input method which assumes a person or if using ROI, though this comes closer and maintains smoothness so long as the ROI does not change, the ROI does eventually change. In short, neither, work if the need is full autonomy; such is the need
#### Version

Latest software being used by dronekit 2.5
#### Platform

[ ] All
[ ] AntennaTracker
[ x ] Copter
[ ] Plane
[ ] Rover
#### Airframe type

Quadcopter
#### Hardware type

That used by 3DR solo

Start by tracing the existing GUIDED-mode set-velocity behavior and comparing the RC-input and ROI paths described in the issue. Done means a documented, autonomous API can specify yaw rate and gimbal attitude rate for smooth dynamic-scene control, with the relevant scenario behavior covered.

Written by the indexing model from the issue text.