usb: samples: Add USB audio Headphones + Microphone sample

This commit adds USB audio sample for Zephyr project.

The sample configures two devices:
- Microphone (2 channels, Fs=48kHz, PCM format)
- Headphones (2 channels, Fs=48kHz, PCM format)

Input data stream is passed to output data stream.

For more details refer to README.rst.

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