cb_cmds: add support for server tags in option definitions
Extend the cb_cmds to support associating option definitions with the server tags, similar to what has been done in #676 (closed). See the design: https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#remote-option-def4-del.
This ticket covers both DHCPv4 and DHCPv6.