cmd/{k8s-operator,containerboot},k8s-operator,kube: reconcile ExternalName Services for ProxyGroup (#13635)

Adds a new reconciler that reconciles ExternalName Services that define a
tailnet target that should be exposed to cluster workloads on a ProxyGroup's
proxies.
The reconciler ensures that for each such service, the config mounted to
the proxies is updated with the tailnet target definition and that
and EndpointSlice and ClusterIP Service are created for the service.

Adds a new reconciler that ensures that as proxy Pods become ready to route
traffic to a tailnet target, the EndpointSlice for the target is updated
with the Pods' endpoints.

Updates tailscale/tailscale#13406

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

cmd/k8s-operator/testutils_test.go

@@ -639,6 +639,14 @@ func removeHashAnnotation(sts *appsv1.StatefulSet) {
 	delete(sts.Spec.Template.Annotations, podAnnotationLastSetConfigFileHash)
 }
 
+func removeTargetPortsFromSvc(svc *corev1.Service) {
+	newPorts := make([]corev1.ServicePort, 0)
+	for _, p := range svc.Spec.Ports {
+		newPorts = append(newPorts, corev1.ServicePort{Protocol: p.Protocol, Port: p.Port})
+	}
+	svc.Spec.Ports = newPorts
+}
+
 func removeAuthKeyIfExistsModifier(t *testing.T) func(s *corev1.Secret) {
 	return func(secret *corev1.Secret) {
 		t.Helper()