usb: samples: Add USB Audio Headset sample

This commit adds USB audio sample to the Zephyr project.

The sample configures one I/O device:
- Headset (2 channels, Fs=48kHz, PCM format)

Input data sstream is passed to output data stream.

For more details refer to README.rst.

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>