[keycloak-dev] Passing login_hint up to IdP when using kc_idp_hint

Hi Keycloak team, I'm working on a system which uses Keycloak as a broker to both OIDC and SAML2.0 IdPs. We are using `kc_idp_hint` for every request and Keycloak is never exposed to the user. The system uses OIDC to connect to Keycloak. We would like to pass a `login_hint` or `subject` upstream to IdPs (depending if it's OIDC or SAML) as we expect to know the user's IdP user name, but this does not work out of the box. I can't see anything in the documentation that would enable it. Is it possible? If so how? Many thanks for any help or pointers you can give. Peter Chamberlin