title: Airflow: how to mark ExternalTaskSensor operator as Success after timeout
body: <p>I have a dag <code>A</code>, which is waiting for some other operators in other dags <code>B</code> and <code>C</code> to download the data, and then performs come computations on it.</p>
  
  <p>It for some operators in dags <code>B</code> and <code>C</code> it takes too long, I'd like to continue without "hanging" operators and use whatever data I received so far.</p>
  
  <p>Thus, I have a timeout, and I'd like to <strong>mark my <code>ExternalTaskSensor</code>s as Success after a given timeout.</strong><br>
  How can I do that?</p>
  
  <pre class="lang-py prettyprint-override"><code># dag A:
  
  wait_for_task_1 = ExternalTaskSensor(
      task_id='wait_B_task_1',
      external_dag_id='B',
      external_task_id='task_1',
      dag=dag,
      timeout=(4 * 3600) # After 4 hours, I want to continue A "as is"
  )
  </code></pre>