Bluetooth: Audio: Split bt_audio_codec_meta_get to cfg and cap

Split the bt_audio_codec_meta_get functions to cfg and cap
variants. This provides a more explicit API where users
do not need to dereference their cfg/cap structs in order
to use this functions.

Furthermore this will also make the get functions more
similar to the upcoming set functions, where the set functions
will require the use of the cfg and cap structs.

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