Builds what tells the partner a session here ended, as a link or a form the sign-out page draws: a signed SAML LogoutRequest, an OpenID Connect RP-Initiated Logout redirect, or a WS-Federation wsignout1.0.
the sign-on session that ended
the sign-out request's base URL
{ ok: true, target }, or { ok: false, why }
Forwards to
FederationSlo.partnerLogoutFor()on the installed instance.