Automatic home realm discovery at the IDP should be possible based on a login_hint coming from the RP via OIDC.
1. User enters his email at a web-portal to start the login flow.
2. email will be used as a login_hint when redirecting to the OIDC service
3. Fediz plugin at OIDC service extracts user domain from email address and sends domain as whr to Fediz IDP.
4. Fediz IDP uses whr parameter to redirect user to its home realm
5. User logs in to his home IDP.