title: How to specify client-specific custom attribute in access token
body: <p>I have a Keycloak setup and I'm using the client_credential flow to create  access tokens for thousands of back-ends.</p>
  <p>I would like to include another custom field in the access token, so next to <code>exp</code> etc., I'd like to have a field <code>publicKey</code>. This field is specific and unique for each of the clients.</p>
  <p>Is that possible using Keycloak?</p>
  <p>EDIT: Clarified that I have many back-ends and each of them has a specific and unique <code>publicKey</code></p>