Support PREF64 for Router Advertisements

[samip5]

Important notices

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• [x] I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• [x] 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.

This request is relevant to IPv6-only networking, as this is one of the prerequisites for deploying IPv6-mostly network.

Describe the solution you like

I would like to be able to specify PREF64 (RFC 8781) option for Router Advertisements.

Describe alternatives you considered

N/A

Additional context

Relevant is RFC 8781