add auth type property to vng config (#13183)

Author: neethirshetty

specification/network/resource-manager/Microsoft.Network/stable/2020-11-01/virtualNetworkGateway.json

@@ -2634,6 +2634,23 @@
           },
           "description": "VpnClientProtocols for Virtual network gateway."
         },
+        "vpnAuthenticationTypes": {
+          "type": "array",
+          "items": {
+            "type": "string",
+            "description": "VPN authentication types enabled for the virtual network gateway.",
+            "enum": [
+              "Certificate",
+              "Radius",
+              "AAD"
+            ],
+            "x-ms-enum": {
+              "name": "VpnAuthenticationType",
+              "modelAsString": true
+            }
+          },
+          "description": "VPN authentication types for the virtual network gateway.."
+        },
         "vpnClientIpsecPolicies": {
           "type": "array",
           "items": {