ManagementPartner: added x-ms-enum for swagger linter (#17961)

Co-authored-by: Ochi <[email protected]>

Author: Ochirkhuyag

specification/managementpartner/resource-manager/Microsoft.ManagementPartner/preview/2018-02-01/ManagementPartner.json

@@ -318,7 +318,11 @@
       "enum": [
         "Active",
         "Deleted"
-      ]
+      ],
+      "x-ms-enum": {
+        "name": "ManagementPartnerState",
+        "modelAsString": true
+      }
     },
     "ErrorResponseCode": {
       "type": "string",