OIDC backchannel logout

[Anderen2]

Is your feature request related to a problem? Please describe.
There is currently no support for OIDC backchannel logout in Authentik. This causes application sessions to be kept active even though the user has signed out from Authentik.

Describe the solution you'd like
Support for the OpenID Connect back-channel logout spec.
This should be incorporated as a optional configuration parameter per OAuth2 provider.

For applications without a backchannel logout specified, or if the backchannel request failed, it would be nice if Authentik could notify the user that they might still be logged in.

Describe alternatives you've considered
A temporary workaround here is configuring the token timeout low.

Additional context
There also exists a OIDC front-channel logout method, however this is not used as often.