cmd/containerboot,cmd/k8s-operator: enable IPv6 for fqdn egress proxies (#12577)

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>

cmd/k8s-operator/testutils_test.go

@@ -319,7 +319,8 @@ func expectedHeadlessService(name string, parentType string) *corev1.Service {
 			Selector: map[string]string{
 				"app": "1234-UID",
 			},
-			ClusterIP: "None",
+			ClusterIP:      "None",
+			IPFamilyPolicy: ptr.To(corev1.IPFamilyPolicyPreferDualStack),
 		},
 	}
 }