Add x-ms-enum extension to keyvault (Azure#17055)

* fix

* make these two sealed enums

Author: ArcturusZhang

specification/keyvault/resource-manager/Microsoft.KeyVault/preview/2021-06-01-preview/keyvault.json

@@ -637,6 +637,10 @@
             "enum": [
               "resourceType eq 'Microsoft.KeyVault/vaults'"
             ],
+            "x-ms-enum": {
+              "name": "VaultListFilterTypes",
+              "modelAsString": false
+            },
             "description": "The filter to apply on the operation."
           },
           {
@@ -655,6 +659,10 @@
             "enum": [
               "2015-11-01"
             ],
+            "x-ms-enum": {
+              "name": "ResourceManagerApiVersions",
+              "modelAsString": false
+            },
             "description": "Azure Resource Manager Api Version."
           },
           {