CRSF: Allow future use of unused bits in the "Arming status byte"

[mha1]

This PR allows future use of the currently unused status bits in the byte we use to communicate the commanded arming status to ExpressLRS modules if we ever have a need for them. The changes are fully backward compatible with the existing V2.11 and V2.12 implementation for communicating the commanded arming status to ExpressLRS modules. It is also compatible with modules using ExpressLRS V3 firmware.

Summary of changes:

• sends status byte with every CRSF 0x16 RC Channels packet. Note: the TBS CRSF V3 specification allows sending extra fields. CRSF V3 Frame Details: "size may be bigger than expected frame of given type. This should not be a reason to count the frame invalid. Frame receiver should just ignore extra fields.". ExpressLRS pre-V4 firmware ignores the field and will use CH5 Arming.
• determines arming method and status using bit 0 and 1 of the status byte

• makes status byte bits 2 to 7 available for future use

This complements the changes made to V4 in ExpressLRS/ExpressLRS#3470 and is fully backwards compatible to the existing implementation