router advertisements: migrate to MVC/API

[fichtner]

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

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

Static PHP pages need to go. Feature requests for the new radvd 2.20 exist.

Describe the solution you like

Migrate the GUI to MVC/API, add relevant features and change the menu location to untie it from ISC DHCP.

Describe alternatives you considered

Adding features now, dealing with ISC out of core and do double work later...

Additional context

Dnsmasq #8329 can do some of these things perhaps, FreeBSD rtadvd too. Not sure what to do long term. At least there is movement in radvd development and it mostly behaves nowadays.