code: public Observable<Void> createAsync(String resourceGroupName, String clusterName, String extensionName, ExtensionInner parameters) {
          return createWithServiceResponseAsync(resourceGroupName, clusterName, extensionName, parameters).map(new Func1<ServiceResponse<Void>, Void>() {
              @Override
              public Void call(ServiceResponse<Void> response) {
                  return response.body();
              }
          });
      }
docs: Creates an HDInsight cluster extension.
  
  @param resourceGroupName The name of the resource group.
  @param clusterName The name of the cluster.
  @param extensionName The name of the cluster extension.
  @param parameters The cluster extensions create request.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable for the request