[keycloak-dev] Adding a custom field to OIDC/SAML provider setup

I would like to add a custom field/property/attribute to an OIDC or SAML provider and I'm looking for a few pointers. The use-case is to have many identity providers configured in Keycloak and prompt the user to enter their email address to determine which IdP to redirect the user. Each IdP would have one email suffix that it provides logins for (this would be the custom field). This is a similar flow to Microsoft's Office 365 and OpsGenie's federated login. Although this could be implemented outside of Keycloak, ideally we could contain this as a custom Rest API added to KC while extending a theme and SPI reusing as much as possible inside Keycloak. Any thoughts/tips are much appreciated.