Add MSP output for controlling DJI VTXs

[BladeScraper-Designs]

Just wondering if this is something that can be done. Unfortunately I'm not one to know how to code, otherwise I'd make an attempt to do it myself. Basically, I wonder if you could output an MSP command to turn on/off low power mode on DJI Air Unit, Vista, O3, etc. Currently if you're doing FPV with airplanes that don't have a flight controller, there is no easy way to control the camera, and on the O3, even disabling low power mode in the goggles doesn't actually increase the output power. The air units must receive an MSP ARM command in order to switch to full power mode. My idea is that you could set MSP_ARM output to be triggered by your choice of RC channel (when using serial input protocols), or optionally just have the MSP_ARM command sent a few seconds after power up automatically. Is this something that can be done? It would be very handy when you want to also use oXs itself for its telemetry functions on a FPV plane.

[mstrens]

In principe, it could be done.
The issue is that there is no UART (nor PIO/SM) free anymore on the rp2040 with all the functionalities provided by oXs.