Bluetooth: Audio: Refactor bt_audio_codec_cfg to flat arrays

Refactor the bt_audio_codec_cfg to use flat arrays to store
metadata and codec specific configurations.

The purpose of this is to make it easier to copy the data
between layers, but also to support non-LTV data for non-LC3
codec configurations.

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