AP_RSSI: Add support for Spektrum Satellite "Fade Counter" to fake RSSI

#### Issue details
Spektrum Satellite Receivers do not support RSSI, but do report a "Fade Counter" that signifies the number of dropped packets. This is documented here: [https://www.spektrumrc.com/ProdInfo/Files/Remote%20Receiver%20Interfacing%20Rev%20A.pdf](https://www.spektrumrc.com/ProdInfo/Files/Remote%20Receiver%20Interfacing%20Rev%20A.pdf)

While it is not a true RSSI value, I would argue that it would still be helpful to support this though the AP_RSSI library. AP_RSSI already supports reciever-protocol specific RSSI, so I think the changes would only need to be implemented in the PX4_firmware layer.

Cleanflight supports this. They have some good discussion here: https://github.com/cleanflight/cleanflight/issues/1658

#### Version
N/A (Enhancement)

#### Platform
[ ] All
[ ] AntennaTracker
[X] Copter
[X] Plane
[X] Rover
[ ] Submarine

#### Airframe type
N/A

#### Hardware type
Pixhawk

#### Logs
N/A

Start with the AP_RSSI library's receiver-protocol-specific RSSI handling and the PX4_firmware layer named in the issue. Read the linked Spektrum PDF and Cleanflight discussion to establish how the Fade Counter should map to fake RSSI, then verify the behavior for Copter, Plane, and Rover.

Written by the indexing model from the issue text.