Bluetooth: CAP: Implement bt_cap_initiator_broadcast_foreach_stream

Implement the function bt_cap_initiator_broadcast_foreach_stream that
allows users to iterate on all BAP streams in a BAP broadcast source.
This can be used to easily get references to other broadcast streams in
the same group.

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