OpenID Connect Back-Channel Logout

https://openid.net/specs/openid-connect-backchannel-1_0.html

This specification defines a logout mechanism that uses direct back-channel communication between the OP and RPs being logged out; this differs from front-channel logout mechanisms, which communicate logout requests from the OP to RPs via the User Agent.

Related authlib ticket: https://github.com/lepture/authlib/issues/560