Bluetooth: CSIP: Add support for optional characteristics

Whenever a CSIS instance is registered with
bt_csip_set_member_register we now properly check if the
set size, rank and lock characteristics should be present.
If not, then remove them from the service declaration.

It is done this way, as it is easier (read: possible) to
remove characteristics from the service declarations,
than it is to add the optional characteristics, as the
optional characteristics may be present in some CSIS
instances, but not in others.

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