Add AllocIPv6 option to allow IPv6 address being used for service registration

[42wim]

Description

Fixes #25627 by adding an extra alloc_advertise_ipv6 option similar to the AdvertiseIPv6Addr with the docker driver config. Seems the simplest way to fix this?

I'll finish the contributor checklist if I get a green light about this PR.

Links

See #25627

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[tgross]

Hi @42wim! It looks like if we pull in the fix in #25997 we only ever need this for dual-stack network interfaces. For IPv6-only we'll automatically advertise the IPv6 address.

We were having a discussion about whether it might make sense to have better default behavior in the dual-stack case and default to IPv6 there. But it doesn't look like there's a good way to do that in a backwards-compatible way.

In any case, I'm 👍 on this general approach. You'll want to include a changelog entry (make cl), tests, and documentation updates as well.

[tgross]

The existing error messages below all refer to address_mode = "alloc" and we're adding a bunch of conditionals here for the IPv6 mode. I think these want to be different cases and maybe there's some helper functions we can factor out.

[42wim]

I've added a helper function getAddressPort

[42wim]

I've added the changelog, tests and documentation update.
Also added the helper function

[aimeeu]

Thanks for updating the docs. I left a tiny suggestion so that the alloc_ipv6 definitions match.

[tgross]

LGTM!

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