Add subnet4-update and subnet6-update commands to subnet-cmds hook [ISC-support #14130]

In order to update an existing subnet, you (currently) have to first delete it and then add it. When making a small change to a large number of subnets, this can create excessive amount of traffic.

Could we please have additional commands to update an existing subnet? This was part of the original design, but we didn't implement it at the time (likely ran out of time)

https://gitlab.isc.org/isc-projects/kea/wikis/designs/commands#24-subnets-management S.7. Kea MAY support the #FF0000 subnet4-update command. S.8. Kea MAY support the #FF0000 subnet6-update command.

From the wiki: Those two commands allow making changes to an existing subnet: changing prefix, prefix length, T1, T2, preferred lifetime, valid lifetime timers, allowed client classes, subnet specific options, and subnet-id values. It also allows modifying pools.