Add Service appProtocol for GKE Gateway support as backend (#751)

### Description
Adding `spec.port.0.appProtocol: "HTTPS"` for compliance with GKE gateways
cf : https://cloud.google.com/kubernetes-engine/docs/how-to/secure-gateway#load-balancer-tls

### Issues Resolved
Fixes #751

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---------

Signed-off-by: Damien HARDY <[email protected]>
Signed-off-by: dhardy <[email protected]>

opensearch-operator/pkg/builders/cluster.go

@@ -634,6 +634,7 @@ func NewServiceForCR(cr *opsterv1.OpenSearchCluster) *corev1.Service {
 	labels := map[string]string{
 		helpers.ClusterLabel: cr.Name,
 	}
+	httpsProtocol := "HTTPS"
 	return &corev1.Service{
 		TypeMeta: metav1.TypeMeta{
 			Kind:       "Service",
@@ -650,6 +651,7 @@ func NewServiceForCR(cr *opsterv1.OpenSearchCluster) *corev1.Service {
 				{
 					Name:     "http",
 					Protocol: "TCP",
+					AppProtocol: &httpsProtocol,
 					Port:     cr.Spec.General.HttpPort,
 					TargetPort: intstr.IntOrString{
 						IntVal: cr.Spec.General.HttpPort,