Support receiving backchannel logout notifications

[sandhose]

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

[hughns]

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

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#2090