Support for OPNsense 'register DNS service ports for unified use across core and plugins'

[Hr46ph]

A search didn't bring up this as an issue or feature being reported earlier, so here goes.

I wasn't sure whether to report as a bug or feature. Since the "issue" was introduced with a change in OPNsense, I figured a feature is better suited.

When running Nextdns cli on OPNsense on port 53, DHCP no longer hands out DNS information to clients if its not aware of a service handling DNS. Basically, services have to register themselves for DHCP to become aware of their presence.

In OPNsense release 23.1.6 this change was made:
opnsense/core@9f6df9e

Clients running on port 53 need to "register" themselves. More info here in the commit:
mimugmail/opn-repo#153

and here the issue reported with adguard and nextdns as examples:
opnsense/core#6513

A workaround (for ipv4) is to manually specify the DNS server IP address in DHCP service.