allow opnsense to give up on getting a prefix delegation from ISP after configurable time

[BPplays]

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
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Is your feature request related to a problem? Please describe.

if my internet goes down it seems like opnsense won't send out router advertisements on interfaces set to Track IPv6 Interface even with a ULA range as an IP alias,

Describe the solution you like

a configurable timeout to wait for a prefix delegation if the timeout is exceeded then just advertise the ULA until the interface gets a GUA. maybe with an option to also advertise the last known prefix delegation from the ISP