dronecan 1140.RCInput: add flags to represent LQ and SNR link statistics

[olliw42]

Adds two flags to allow representing LQ and SNR link stats.
As suggested in ArduPilot/ardupilot#28295.
Adds a possible mapping from RSSI vlaues onto the scaled values. It is motivated by https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_RCProtocol/AP_RCProtocol_CRSF.cpp#L602-L610 (with slight adaption).

Also adds a note on the rc channels mid value to clarify the representation of the rc channels values.

[olliw42]

EDIT: I "discovered" that ArduPilot's OSD wants rssi in dBm and not rssi in 0..255, and I couldn't determine a backwards compatible method of using rssi. So added RSSI_DBM option.

[olliw42]

question:

would it be acceptable to carry TWO data in one transmission of the quality field?

the specific use case would be to carry both LQ and the active_antenna in quality. LQ is from 0..100, so the 8th bit is not used, and could be used to carry the bit required for active_antenna.
this would be benefitial, since the data has to be send round robin, so one slot less would be needed just for that single bit.
?