Bluetooth: BAP: Shell: Modify bcast sink streams to use shell_stream

Modify the broadcast_sink_streams to be an array of shell_stream
instead of bt_bap_stream, so that it can be used in a similar way
as the rest of the streams.

To help the transition, a new helper function,
bap_stream_from_shell_stream, was added.

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