add motion_frame::get_combined_motion_data() for DDS Motion streams

[maloel]

No API existed to get the combined motion data.
The previous API in rs2::motion_frame is get_motion_data() returning rs2_vector which ASSUMES the data type. This should be used with RS2_FORMAT_MOTION_XYZ32F.
I did the same: added get_combined_motion_data() returning rs2_combined_motion. This should be used with RS2_FORMAT_COMBINED_MOTION.
I.e., it is up to the user to call the right one.

Added Python API.

Tracked on [RSDEV-2243]

[maloel]

I pushed a change to the comments in rs_frame.hpp, to make it clearer when to use either function.

[maloel]

Just for reference, an example of usage:

def frame_callback( frame ):
    global total_gravity, n_gravity
    m = frame.as_motion_frame()
    if m:
        md = m.get_combined_motion_data()
        log.d( md )
        la = md.linear_acceleration
        total_gravity += math.sqrt( la.x * la.x + la.y * la.y + la.z * la.z )
        n_gravity += 1