Adding FrSky Telemetry downlink

Currently there are 3 commonly used telemetry protocols: mavlink (mode 1), native Frsky (mode 4) & FrSky passthrough (mode 10).
The usage of all requires both uplink and downlink: uplink for requesting data while downlink for receiving the answers.
By implementing another telemetry mode, that just send the sensors data without being queried, all telemetry data can be displayed easily upon OpenTx transmitter
The suggested telemetry mode could be the well know FrSky passthrough which have many supporting scripts such as the fantastic @yapuu.
This way, any datalink supporting 57600 baud rate can be used for telemetry whereas the conversion to SPORT can be done by simple hardware within the transmitter.
Furthermore, the remine unused uplink can be used for other usages (controling, extra channels etc.).
This issue came out in the following thread by @schwabe
https://github.com/opentx/opentx/issues/6388

Start by reading the linked OpenTX issue and the existing telemetry modes 1, 4, and 10. Determine the ArduPilot telemetry entry points and data path for an unsolicited FrSky passthrough downlink at 57600 baud. Done means sensor data is sent without uplink queries and can be displayed by OpenTX-compatible receivers while the uplink remains available for other uses.

Written by the indexing model from the issue text.