cb_cmds: add support for server tags in global options

Similar to #676 (closed), we need to extend the cb_cmds: remote-option4-global-X commands and appropriate unit tests to facilitate the server tags. See the examples in the design: https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#remote-option4-global-del.

This ticket covers both DHCPv4 and DHCPv6.