Bluetooth: BAP: Add target latency and phy to codec_cfg

The codec configuration operation for unicast (ASCS) contain
both a target PHY and a target latency value that was previously
just hardcoded by the client and unavailable by the server.

This commit adds them to the bt_audio_codec_cfg struct so that
applications can both set and get these values.

The default values used by BT_AUDIO_CODEC_CFG use the same
values as the client would previously set.

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