title: Add unique key of my own to slack event requests
body: <p>I created a slack app with distribution (multiple clients)
  and I subscribe to events several events like a message posted in a channel.</p>
  
  <p>Beside the <code>team_id</code> <a href="https://api.slack.com/events-api" rel="nofollow noreferrer">field</a> for identifying the client team - is there an ability to put my own key for a client?</p>
  
  <p>Thank you.</p>