Bluetooth: Audio: Add bt_audio_data_get_val

Add a generic function to retrieve any data based on an
assigned numbers type. This function can in theory be
used for any data type, and not just LE Audio types, but
since it relies on bt_audio_data_parse it was made
specificially for LE Audio.

The function can be used in cases where
bt_audio_codec_cfg_get_val, bt_audio_codec_cfg_meta_get_val,
bt_audio_codec_cap_get_val or
bt_audio_codec_cap_meta_get_val (or their derivation)
are not easily applicable due to the type constrainst.

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