Provide mecanism to stop rf output

[satfan52]

This ticket was originally opened in nanovna-saver but it is not implementable without a firmware change. Please refer to the original ticket for understanding the rationale for this change and why this change can not be implemented in nanovna-saver

NanoVNA-Saver/nanovna-saver#128

The idea is that nanovna V1 would continue to sweep continuously as it does currently. However there could be an option in the firwmare to allow users of nanovna-saver to cut/stop the rf-output when the user does not request nanovna-saver to sweep.