Send client_id in logout request sent by OIDC RP
The OpenID Connect specification about logout has been updated and now client_id
is a possible parameter: https://openid.net/specs/openid-connect-rpinitiated-1_0.html#RPLogout
Keycloak has recently changed its behavior to require either client_id or id_token_hint. We already send id_token_hint if ID Token is stored in session (option disabled by default). We should provide client_id
each time, and id_token_hint
if ID Token is available.