cmd/containerboot,cmd/k8s-operator: enable IPv6 for fqdn egress proxies
Don't skip installing egress forwarding rules for IPv6 (as long as the host
supports IPv6), and set headless services `ipFamilyPolicy` to
`PreferDualStack` to optionally enable both IP families when possible. Note
that even with `PreferDualStack` set, testing a dual-stack GKE cluster with
the default DNS setup of kube-dns did not correctly set both A and
AAAA records for the headless service, and instead only did so when
switching the cluster DNS to Cloud DNS. For both IPv4 and IPv6 to work
simultaneously in a dual-stack cluster, we require headless services to
return both A and AAAA records.
If the host doesn't support IPv6 but the FQDN specified only has IPv6
addresses available, containerboot will exit with error code 1 and an
error message because there is no viable egress route.
Fixes#12215
Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>