Sub: allow ignoring axes in MANUAL_CONTROL

[ES-Alexander]

Summary

Testing (more checks increases chance of being merged)

• Checked by a human programmer
• Tested in SITL
• Tested on hardware
• Logs attached
• Logs available on request
• Autotest included

Description

Allows programmatic users to provide MAVLink MANUAL_CONTROL messages without needing to specify valid values/overrides for all primary motion axes.

Fixes the well-defined part of #8818. The Z limits aren't consistent between vehicle types, and last I recall were established as a legacy issue with the current system that's begrudgingly accepted as not changing any time soon.

If merged, closes #32639 as not needed.

[peterbarker]

It would be rather nicer if you could move to void GCS_MAVLINK::manual_override(RC_Channel *c, int16_t value_in, const uint16_t offset, const float scaler, const uint32_t tnow, const bool reversed) instead - all the other vehicles honour the ignore value because they go through this method.

i.e. fix the problem by removing code rather adding code....

[ES-Alexander]

In light of improved axis configuration in a GCS, and factoring in the low estimated usage of roll-pitch toggling, I'm leaning towards removing the feature from the firmware. That should simplify the transition to consolidating with the manual_override library function, and the firmware code more generally, while also avoiding needing some redundant checks.

[ES-Alexander]

Re-marking as ready for review, so it can be compared against #32639 without the bias that one is marked as draft.