support unicam / libcamera frame sources

[wherget]

Sibling issue to showmewebcam/showmewebcam#186.

Expected Behavior

on systems which use the unicam interface, uvc-gadget should be able to control the filter pipeline

Current Behavior

(not checked, but I'm assuming most V4L controls don't work on the unicam video device, as sensor control is done over libcamera)

Possible Solution

implement a libcamera control mapping backend

[RoganDawes]

Another vote for this, please. I bought a couple of Arducam modules, and would love to use them with showmewebcam.