Bluetooth: BAP: Shell: Add support for USB audio in

Add support for receiving audio data from e.g. a PC
over USB and LC3 encode it before sending it
on BAP audio streams.

This refactores the entire TX path, as it has moved
from only support the sine wave generator, to also
supporting USB.

The encoding and sending of data is now in it's own
thread, instead of relying on the system workqueue thread
and k_work items.

Several other refactors have taken place to reduce lines
of codec (such as the introduction of the bap_foreach_stream
function.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>