rfc2136: Support ipv6 prefix delegation update for static suffix

[ovv]

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Is your feature request related to a problem? Please describe.

My ISP delegate a /56 but can't commit to it being fully static. Servers behind the firewall get assigned static ipv6 suffix and I'd like to add those address to an external DNS server.

Describe the solution you'd like

I'd love for the rfc2136 plugin to support prefix delegation and static suffixes. When the monitoring interface IP change it would only update the prefix part of the IPs.

On services_rfc2136_edit.php, and only for AAAA record type, add a suffix field. When executing, the plugin would concatenate the delegated prefix and the new suffix field to build the complete IP.

Describe alternatives you've considered

Running an nsupdate cron on one or all servers so they can configure their IPs.

Additional context

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