Adding missing customerName property to fix S360 swagger correctness (#23948)

Co-authored-by: Umang Shah <umangshah@microsoft.com>

Author: us6193

specification/billing/resource-manager/Microsoft.Billing/stable/2021-10-01/billingSubscription.json

@@ -736,6 +736,11 @@
           "type": "string",
           "readOnly": true
         },
+        "customerName": {
+          "description": "The name of the customer for whom the subscription was created. The field is applicable only for Microsoft Partner Agreement billing accounts.",
+          "type": "string",
+          "readOnly": true
+        },
         "displayName": {
           "description": "The name of the subscription.",
           "type": "string"