Bluetooth: Audio: Refactor ISO allocation endpoints

Modify how the ISO channels are allocated for the audio streams,
as well as how the CIG is allocated for the unicast group.

This fixes an issue where the unicast group could not be
fully allocated before _after_ the bt_audio_streams had been
configured by bt_audio_stream_config, thus making it impossible
to create a unicast group before the connections have been established.

This leaves us with 3 basic data types:
1) Streams allocated by the application
2) Endpoints that represent ASEs
3) Audio_iso which is used to couple streams and ISO channels.

The Unicast Group for the unicast client will now have the same
lifetime as the ISO CIG for the central.

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