title: AWS API Gateway: Add header if response is Unauthorized
body: <p>I setup an API Gateway in AWS that uses custom authorizers to implement an OAuth2 flow. It works fine. When the user is not authorized they get a 401 Authorized response. That is correct as well, but I would like to add a header that gives the client the endpoint where it can get the token. Something like <code>AuthorizeUrl: url</code> </p>
  
  <p>How can I add this header to my response?</p>