Allow configure post_logout_redirect_uri for confidential client

A confidential client integrates with Authgear via the authorization code flow. If the client is a web application, then it may want to log the user out from both the server and Authgear. OIDC defines `end_session_endpoint` for this purpose. We have supported this already. But `post_logout_redirect_uris` are not visible to a confidential client.