Add SNR-driven dynamic coding rate selection for repeater retransmits #638

Job	Run time
build (LilyGo_Tlora_C6_repeater_)	2m 4s
build (wio-e5-mini_repeater)	1m 38s
build (RAK_4631_repeater)	2m 13s
build (RAK_4631_companion_radio_ble)	2m 21s
build (Heltec_v3_companion_radio_ble)	2m 52s
build (Heltec_v3_repeater)	2m 51s
build (PicoW_repeater)	2m 47s
build (Heltec_v3_room_server)	2m 40s
build (RAK_4631_room_server)	2m 12s
	21m 38s

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