Add proxy_ssl_* directives

Add support for backends which require client certificate (eg. NiFi)
authentication. The `proxy-ssl-secret` k8s annotation references a
secret which is used to authenticate to the backend server. All other
directives fine tune the backend communication.

The following annotations are supported:
* proxy-ssl-secret
* proxy-ssl-ciphers
* proxy-ssl-protocol
* proxy-ssl-verify
* proxy-ssl-verify-depth

internal/ingress/controller/store/store.go

@@ -684,6 +684,7 @@ func (s *k8sStore) updateSecretIngressMap(ing *networkingv1beta1.Ingress) {
 	secretAnnotations := []string{
 		"auth-secret",
 		"auth-tls-secret",
+		"proxy-ssl-secret",
 	}
 	for _, ann := range secretAnnotations {
 		secrKey, err := objectRefAnnotationNsKey(ann, ing)