title: Use webhook to permit users access AWS S3 objects
body: <p>I want to use AWS S3 for saving files of my users. Users may be permitted to access other's dynamically.
  I don't want to use IAM policies (or similar ways) for controlling access of users.
  Is it possible for S3 to invoke my API endpoint with user's information and asks for permission?</p>
  
  <p>PS. Users are using credentials that I have gave them.</p>