add client class selection to pool

As described here: https://kea.readthedocs.io/en/kea-2.4.1/arm/dhcp4-srv.html#pool-selection-with-client-class-reservations you can require a client-class membership in the pool {} statement inside a pools [] block. Now that client-classes exist, we should allow selection of a client class in each pool {} statement. This should probably include the special classes of KNOWN and UNKNOWN.