samples: usb: add UAC2 implicit feedback sample

The sample illustates how to achieve bidirectional asynchronous audio.
Implicit feedback is the only way to achieve asynchronous USB headset
on devices that have only one isochronous IN and one isochronous OUT
endpoint.

The sample implements stereo playback and mono recording. While it would
be possible to have stereo recording, the commonly available headsets
are mono only. The number of channels can differ between audio sink and
source because the only requirement is that the two run on same clock.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>