samples: usb: add UAC2 explicit feedback sample

The sample illustrates how to calculate feedback value based on I2S
sample clock measurement or indirect I2S FRAMESTART and USBD SOF.
The sample is currently only supported on nrf5340dk_nrf5340_cpuapp
target because the feedback measurement uses target specific
peripherals.

While it should be possible to perform feedback value calculation
entirely in software (possibly with some additional filtering due
to software scheduling jitter), the I2S API does not provide necessary
timestamps.

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