Add support for SHUTDOWN message from Pico, and firmware to send it

[ec1oud]

The firmware is from https://github.com/ec1oud/zbitxfrontpanel/commits/shutdown/

[mikerodrigues]

@ec1oud do I need to do anything besides upload your included uf2 file to take advantage of this? I've been really sour about the lack of support for this and would love to use your improvements.

*Realized I had to copy the sbitx binary over too.

For anyone doing this and not wanting to pull the whole repo, just update with the uf2 as per instructions: https://www.sbitx.net/index.php/updating-the-zbitx/

you can run killall sbitx && cp ./zbitx/sbitx ./sbitx/sbitx after sshing in. Then power cycle it and it should all work.