Classifications Changes 4.0 Change 'client-class' in shared-network, subnet, and pool to a list "client-classes"

Guard classes for shared network, subnet, and pool are limited to a single class. This would replace the 'client-class' element with a list, 'client-classes' as described here: https://gitlab.isc.org/isc-projects/kea/-/wikis/Designs/Potential-Classification-Changes#4-change-client-class-in-shared-network-subnet-and-pool-to-a-list-client-classes