Bluetooth: Audio: CSIS: Refactor CSIS client discover API

The API will now take a simple bt_conn instead of a
set member struct. This is due to the set member struct
being moved to the stack. The reasoning for this is that
having the struct in the stack, rather than in the
application, the stack itself can initiate the CSIS
discovery which is useful if the remote device
has a service that includes a CSIS.

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