common.xml: add fuel_pressure ext. to EFI_STATUS

[robertlong13]

Add support for reporting fuel pressure in the EFI_STATUS message. Fuel pressure is already measured and logged by multiple EFI backends, and it can be a useful measurement to report to the GCS for diagnosing problems on the ground.

kPa chosen as the unit to match the two other pressure fields in the message, though this is a departure from other pressure fields in other messages.

We could potentially add other missing fields at this time too, but this one is the most useful.

[peterbarker]

@hamishwillee FYI. We'll discuss this at DevCall tomorrow and then PR upstream

[hamishwillee]

Thanks @peterbarker - sounds good to me. As an aside, do you remember why we didn't enumerate EFI_STATUS.health. If not used perhaps we should make a comment on that too.

[magicrub]

@robertlong13 merged! Sounds like @hamishwillee also likes this so can you please make a matching "upstream" PR https://github.com/mavlink/mavlink

[hamishwillee]

Yes I like this, but it should have been created in mavlink/mavlink first and then filtered down to Ardupilot/mavlink - because it is in common. No guarantees that it will be loved by the broader community, who have much more expertise than me.