Fix race condition in ApiClient::invokeApi() (#1580)

Author: arvindkrishnakumar-okta

api/src/main/resources/custom_templates/ApiClient.mustache

@@ -1087,7 +1087,7 @@ protected List<ServerConfiguration> servers = new ArrayList<ServerConfiguration>
  * @return The response body in type of string
  * @throws ApiException API exception
  */
- public <T> T invokeAPI(
+ public synchronized <T> T invokeAPI(
  String path,
  String method,
  List<Pair> queryParams,