Support receiving backchannel logout notifications

[matrixbot]

This issue was originally created by @sandhose at matrix-org/matrix-authentication-service#2090.

We should support getting backchannel logout notifications from upstream OIDC providers and log out the sessions when we get one. What happens exactly in terms of which session gets logged out is still TBD

[matrixbot]

This comment was originally posted by @hughns at matrix-org/matrix-authentication-service#2090 (comment).

Current Synapse capability is at https://matrix-org.github.io/synapse/latest/openid.html#oidc-back-channel-logout which is an opt-in feature.