title: Email claim overrides username in keycloak
body: <p>I've written a custom OIDC-Provider and connected it to keycloak.
  After the first login in via my provider, the user gets asked to enter a username, email, first name and last name (first-broker-login-flow).</p>
  
  <p>Usually the username is prefilled with what is in the 'sub' claim.
  But when sending an 'email' claim, the username is prefilled with the email.</p>
  
  <p>How can I prefill the username with something else than my email, while also sending an 'email' claim?</p>
  
  <p>So far I've tried sending a 'name', a 'username' and a 'preferred_username' claim  -  no success.</p>
  
  <p>I figured out that the IdpReviewProfileAuthenticator checks for isRegistrationEmailAsUsername().
  So I've deactivated "Login with email" in my realm settings  -  no success.</p>
  
  <p>Is there any way for keycloak to not use the 'email' claim for the username?</p>