Logout endpoint support

[farsett]

Description

I use Keycloak as an OIDC provider, but I have encountered an issue where specifying end_session_endpoint in the OIDC Headplane configuration does not work. I want the user to be automatically logged out of their Keycloak account when they log out of Headplane, but this is not happening.
At the moment, I have simply set a short session lifetime in Keycloak, but it would be very convenient to trigger a redirect on logout. Thank you in advance!