usb: device_next: uac2: Support multiple sample rates

Add callbacks for setting and getting the sample rate. The callbacks are
optional if all Clock Source entities support only one sample rate.

This commit results in working High-Speed operation with Windows UAC2
driver when the Clock Source is host-programmable. Windows UAC2 driver
won't work if setting sample rate fails even if Clock Source supports
only one sample rate.

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