title: specify user id when creating user in keycloak
body: <p>I'm investigating a migration process from a legacy system into keycloak. Based on some of the ideas here: <a href="https://github.com/Smartling/keycloak-user-migration-provider" rel="nofollow noreferrer">https://github.com/Smartling/keycloak-user-migration-provider</a> we're looking to create user accounts in keycloak at the point of login by looking up user credentials from some dedicated endpoints on our legacy system.</p>
  <p>As part of this, I need the user ID to remain the same as it was in the legacy system.</p>
  <p><strong>Is it possible to create a user with a specified ID rather than relying on keycloak to auto-generate it?</strong></p>