Bluetooth: Audio: Add CAP stream

Add a new stream object, bt_cap_stream, which is an extension
of the BAP bt_audio_stream. The purpose of this stream
is that we can extend the data stored in the BAP stream for
CAP usage, as well as making it more explicit what type
of stream should be used for CAP.

The callbacks will be extended for CAP in specific use cases,
e.g. when starting one or more unicast audio streams.

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