(ARMSubscription) fixes Azure/azure-sdk-for-go#18620 (#19870)

* (ARMSubscription) fixes Azure/azure-sdk-for-go#18620

fixes #18620

The subscriptions Model struct includes a number of fields, but not all the fields available in the REST API according to the documentation. Specifically, the following fields are missing in the Go struct:

tenantId - The subscription tenant ID.
tags - The tags attached to the subscription.

Note that these fields are returned by the API and are documented here: https://docs.microsoft.com/en-us/rest/api/resources/subscriptions/list?tabs=HTTP#subscription

* Update subscriptions.json

specification/subscription/resource-manager/Microsoft.Subscription/stable/2016-06-01/subscriptions.json

@@ -240,6 +240,18 @@
             "modelAsString": false
           }
         },
+        "tenantId": {
+          "readOnly": true,
+          "type": "string",
+          "description": "The tenant ID. For example, 00000000-0000-0000-0000-000000000000."
+        },
+        "tags": {
+          "description": "Tags for the subscription",
+          "type": "object",
+          "additionalProperties": {
+            "type": "string"
+          }
+        },
         "subscriptionPolicies": {
           "$ref": "#/definitions/SubscriptionPolicies",
           "description": "The subscription policies."