add --ip6 flag to podman create/run

[Luap99]

What this PR does / why we need it:

Add the --ipv6 flag to podman create/run and pod create. We support the
--network name:ip6=<ip> syntax now but for docker compat we should also
support the --ip6 flag.
Note that there is no validation if the ip is actually a v6 or v4 address
because the backend does not care either.

How to verify it

Which issue(s) this PR fixes:

Fixes #7511

Special notes for your reviewer:

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Luap99

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [Luap99]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[vikas-goel]

If this PR is enabling static IPv6 support, will be good to update podman create/run man pages too.
Will it enable static IPv6 support for macvlan networks?

[TomSweeneyRedHat]

All kinds of test unhappiness. Code generally LGTM

[Luap99]

@vikas-goel static ipv6 support was added in #12534
This PR just adds the --ip6 flag. The docs are also updated.

[Luap99]

@vikas-goel Macvlan should work too, that might depend if CNI supports this but it will definitely work with the new network stack netavark.

[Luap99]

@containers/podman-maintainers PTAL

[rhatdan]

/lgtm

[vikas-goel]

When will 4.0 be released?

[vikas-goel]

When will podman static IPv6 be included in RHEL8?

[Luap99]

I think the goal is to release 4.0 in February and to get it into RHEL 8.6/9.0

[vikas-goel]

Thank you @Luap99. Did you mean both RHEL 8.6 and 9.0?

[rhatdan]

That is a goal, but we could definitely miss it and it would be RHEL 8.6.0.2 and 9.0.0.2

[vikas-goel]

Thank you @rhatdan