Bluetooth: CAP: Move initiator/commander common code to cap_common

The CAP Initiator and the CAP Commendar share quite a lot of
functionality in terms of discovery and performing set-based
procedures.

The code for handling most of this has been moved to
cap_common.c to avoid duplicating code.

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