Add selection of client classes to reservation

It is possible to add hosts to a class inside a reservation as noted here: https://kea.readthedocs.io/en/kea-2.4.1/arm/dhcp4-srv.html#reserving-client-classes-in-dhcpv4 Make this a possibility in the reservation. If the reservation is global, check, but I think early-global-reservations-lookup is required, see note in this section: https://kea.readthedocs.io/en/kea-2.4.1/arm/dhcp4-srv.html#client-classification-in-dhcpv4