title: Keycloak retrieve custom attributes to KeycloakPrincipal
body: <p>In my rest service i can obtain the principal information after authentication using</p>
  <pre><code>KeycloakPrincipal kcPrincipal = (KeycloakPrincipal) servletRequest.getUserPrincipal();
  </code></pre>
  <p>statement.</p>
  <p>Keycloak principal doesn't contain all the information i need about the authenticated user.
  Is it possible to customize my own principal type?
  On the keycloak-server-end I've developed a user federation provider. I saw that UserModel makes possible to add a set of custom attributes to my user.</p>
  <p>Is it possible to insert my custom principal in that code?</p>
  <p>Is it possible to retrieve this attributes from keycloak principal?</p>
  <p>What is the way?</p>