AP_OpticalFlow: Use flow rates and add ground distance field in MAVLink driver

[snktshrma]

Closes #29131 #30019 . Updated OpticalFlow MAVLink driver for improved accuracy:

• Replaced flow_x and flow_y (int16_t) with flow_rate_x and flow_rate_y (float) to enhance accuracy.
• Added ground_distance field to transmit ground distance over MAVLink when available.

Edit:

• Added EK3_FLOW_HGT_SRC param to select between Optical flow's ground distance (currently only mavlink and/or) or default.

[rmackay9]

Hi @snktshrma,

We're still interested in this PR by the way. I'd like to try implementing optical flow using a downward facing camera gimbal.

[snktshrma]

Hi @rmackay9 sir! Yes I am sorting the failing tests and will push the final updates to be merged asap and will try to add an example for optical flow through gimbal as well