Send RTC time from the TX backpack to the VRX backpack over ESP-NOW

[bob9]

Summary

Syncs the VRX backpack's clock (and, on HDZero, the goggles' RTC) from the TX backpack, so DVR recordings get correct timestamps without the pilot ever opening the backpack WiFi page.

• The TX backpack seeds its local clock when it receives MSP_ELRS_BACKPACK_SET_RTC on its serial port (in addition to the existing WiFi NTP path).
• When the VRX backpack requests its startup packet (MSP_ELRS_REQU_VTX_PKT), the TX backpack replies with the current time over ESP-NOW — only when its clock has actually been set, and only on request (no periodic resync).
• The VRX backpack sets its own system clock from the packet and forwards the time to the goggles over serial, reusing the existing SetRTC() path that previously only ran during WiFi NTP sync. On modules without an RTC serial protocol this is a no-op via the existing ModuleBase::SetRTC stub.

Uses the existing MSP_ELRS_BACKPACK_SET_RTC (0x030E) opcode — this only adds a new transport for it. Additive only; devices that don't handle it ignore the message.

Related PRs

• Add Sync Backpack Time button to set the backpack RTC from the handset Lua-Scripts#18 — adds a Sync Backpack Time button to the ELRS Lua script that sends the handset's date/time to the TX module (field id 0x3C), seeding the TX backpack clock that this PR distributes.
• Forward MSP_ELRS_BACKPACK_SET_DVR_NAME to the goggles #232 — forwards the DVR recording name to the goggles; recordings named by that feature get correct timestamps from this PR's time sync. Both PRs were split out of Send RTC time and DVR recording name from the TX backpack to the goggles #230.
• Name DVR recordings after the race via MSP_SET_DVR_NAME (0x030F) hd-zero/hdzero-goggle#613 — goggle PR that names DVR recordings after the race; this PR's time sync gives those recordings correct timestamps.
• Send handset time to the TX backpack to set its RTC ExpressLRS#3701 — TX-module firmware PR that sends MSP_ELRS_BACKPACK_SET_RTC to the TX backpack, seeding the clock this PR distributes.