AP_OpticalFlow: mavlink driver should consume OPTICAL_FLOW message's flow_rate_x, flow_rate_y

[rmackay9]

The OpticalFlow_MAV driver consumes the OPTICAL_FLOW message's "flow_x" and "flow_y" fields which are int16_t. Instead we should consume the "flow_rate_x" and "flow_rate_y" extension fields which are floats and will thus give better accuracy.

We should also consume the "ground_distance" field

We should also consume the OPTICAL_FLOW_RAD. I'm not sure why both messages exist but perhaps this one was created specifically for flow sensors to send their readings to the autopilot while OPTICAL_FLOW was meant for autopilots to send their flow data to the GCS? I'm not sure why we'd need to separate the uses though