Add support for RADIO_LINK_STATS

[olliw42]

This is the second step in adding MAVLink receiver support, which is about radio link statistics. The first step was adding the MAVLinkRadio receiver class, which was about handling the RADIO_RC_CHANNELS mavlink message, and which was recently merged (#26356).

The PR is dependent on ArduPilot/mavlink#354, which brings in the definition of the RADIO_LINK_STATS message.

The PR will thus not work at this time, but is intended to show where things may go, and to spur the discussion on the code.

The PR does these things:

• adds handling of RADIO_LINK_STATS to the AP_RCProtocol library. In this library the LQ and rssi fields are digested and made available via the usual ways.
• adds logging. Since these are more than ca 7 data items to log, two new log items are created for the logging, RDRX and RDTX.
• adds handling of RADIO_LINK_STATS to the GCS library. It reads the message and forwards it to the AP_RCProtocol library and does the logging.