message: Update managedInstances.json
  Adding two new ManagedInstance properties
diff: "x-ms-mutability": [
  "create"
  ]
  + },
  + "publicDataEndpointEnabled": {
  + "description": "Whether or not the public data endpoint is enabled.",
  + "type": "boolean"
  + },
  + "proxyOverride": {
  + "description": "Proxy override of the managed instance.",
  + "type": "string"
  }
  }
  },
  