samples: usb: add new UVC sample

Following the addition of USB Video Class, this adds a sample that makes
use of the zephyr,camera chosen node of any board to stream the video
source to the host. A snippet video-sw-generator can be used to test
and debug devices without a zephyr,camera chosen node.

Signed-off-by: Josuah Demangeon <me@josuah.net>