usb_audio module and USBMicrophone class

[FoamyGuy]

The API is modeled after usb_video. Enable the audio endpoint inside boot.py and hook it to existing audio source APIs in code.py.

All of my testing was on rp2040 based devices, so that is the only port that it's enabled in currently.

One open question: while working on audioi2sin it was suggested to make it one word i.e. not audio_i2sin, the existing usb_something modules are all split into two words with an underscore so I've done the same with usb_audio for consistency to the rest of them. Perhaps it should be introduced as usbaudio instead though, especially if we would want to eventually rename the others.

[dhalbert]

I think usb_audio is fine, given usb_midi, etc.