Honor upstream OIDC back-channel logout and kill the local session

## What

authentik as OAuth2/OIDC relying party must accept back-channel logout from the upstream OP and kill the local session.

## Want

- Authentik as OAuth2/OIDC relying party must accept back-channel logout from the upstream OP
- Kill the local session

## Ideas this came from

- [#16484 Backchannel Logout for OAuth2 source](https://github.com/goauthentik/authentik/issues/16484) — I have a setup with multiple authentik instances with are connected over federations. When logging out from the main instances the session in the other instance
- [#19707 Can/Could Authentik serve as a Relying Party (RP) and receive Back Channel Logouts?](https://github.com/goauthentik/authentik/issues/19707) — Is your feature request related to a problem? I have an OpenID OIDC provider that Authentik is properly validating users against. My issue is invalidating the A

No files or tests are identified in the issue. Start by locating the OAuth2/OIDC relying-party logout handling and determine how upstream back-channel logout can identify and invalidate the local session; done means authentik accepts the upstream logout and the corresponding local session is killed.

Written by the indexing model from the issue text.