Add selection of options to subnet

As shown in this section, https://kea.readthedocs.io/en/kea-2.4.1/arm/dhcp4-srv.html#standard-dhcpv4-options , option-data can be (and often is) specified in the subnet block. re-use the option specification mechanism to make adding options to the subnet block possible. In Kea, it is also possible to specify options with the pools, but I think we will ignore that in this simplified kea-quick-config as I don't believe it is that common to do so.