Add CMS support on TX for ELRS

[SteveCEvans]

This PR throttles the sending of chunks of DisplayPort data to one every 75ms to prevent overrun on ELRS telemetry channel. Tested at 1:2 to 1:128 Telem Ratios at 500Hz.

Note that this only works at 333Hz and above or D250 and above. Slow update rates don't have the telemetry bandwidth required, but surprisingly telemetry ratio has no apparent impact.

@AlessandroAU if there is a better way to apply flow control to the telemetry channel than this 75ms delay, please let me know.

[howels]

Telemetry ratio doesn't have an impact cos the telemetry rate automatically switches into 1:2 when encapsulated MSP is sent

[blckmn]

AUTOMERGE: (FAIL)

• github identifies PR as mergeable -> FAIL
• assigned to a milestone -> PASS
• cooling off period lapsed -> PASS
• commit count less or equal to three -> PASS
• Don't merge label NOT found -> PASS
• at least one RN: label found -> PASS
• Tested label found -> FAIL
• assigned to an approver -> PASS
• approver count at least three -> FAIL

[SteveCEvans]

Telemetry ratio is set to 1:2 for MSP and CMS traffic, which explains why it had no impact.

Now sends a CRSF ping to the RX and checks the returned device info to identify ELRS receivers and only adds DisplayPort chunk delay if necessary.

[SteveCEvans]

Need to fix the unit tests…

[howels]

Tested and working on MatekH743 with BetaFPV eLRS Nano RX @500hz and EdgeTX 2.8 on Jumper T-Lite

Failed to work on JHEH743AIO board, flashed twice but no USB connection afterwards. Flashed master and all worked again.

[haslinghuis]

@SteveCEvans some builds are failing (probably fixed if we merge #12394 and rebuild

[github-actions]

Do you want to test this code? Here you have an automated build:
Assets
WARNING: It may be unstable. Use only for testing! See: https://www.youtube.com/watch?v=I1uN9CN30gw for instructions for unified targets!