Bluetooth: Audio: Add bt_audio_dir_str to improve logging of dir

The direction of a stream/endpoint/parameter has just been
logged as a unsigned integer. This commits adds a
value -> string internal function that would log
BT_AUDIO_DIR_SINK as "sink" and BT_AUDIO_DIR_SOURCE
as "source".

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