AP_Mount: DJI RS2/3 should support "follow" during RC rate control

[rmackay9]

AP supports the DJI RS2/RS3 hand held gimbals as described here on the wiki.

Unfortunately the driver (and perhaps the gimbal itself) does not support yaw rate control in body-frame (aka "follow"). Yaw rate control is always in earth-frame (aka "lock"). The missing support can be seen here in the send_target_rates() which simply does not have a "yaw_is_ef" argument. We can see at the bottom of the script that we get the yaw_is_ef from the C++ stub but we don't use it.

This issue was identified in this Copter-4.5 discussion: https://discuss.ardupilot.org/t/ap-support-for-dji-rs2-rsc2-rs3-gimbals-maybe/91830/38