The seam for delivering a Back-Channel Logout logout_token to a Relying
Party (OpenID Connect Back-Channel Logout 1.0 §2.5).
An implementation POSTs the token to the RP's backchannel_logout_uri as
application/x-www-form-urlencoded with a single logout_token parameter,
and reports whether the RP accepted it. Delivery is best-effort by design:
the end-session endpoint logs failures but never blocks the user's logout on
an unreachable RP. The default implementation is
AttestoPhoenix.BackChannelLogout.Req; a host overrides it with
logout: [http_client: MyDeliverer].
Summary
Callbacks
POST logout_token to backchannel_logout_uri. Returns :ok when the RP
responded with a 2xx, {:error, reason} otherwise.