Bluetooth: Audio: Move most of unicast client-only code from stream.c

Some operations such as Config, QoS, Enable and Stop
are only available for the unicast client, but since
they share the bt_audio_stream prefix, we leave the
agnostic part of it in stream.c, and move the actual
implementation to unicast_client.c.

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