Default AttestoPhoenix.BackChannelLogout deliverer, built on Req.
POSTs the logout_token to the Relying Party's backchannel_logout_uri as
application/x-www-form-urlencoded (OpenID Connect Back-Channel Logout 1.0
§2.5). Redirects and retries are disabled and the timeout is short: logout
delivery is best-effort and must not let a slow or hostile RP stall the
end-session response. A 2xx is success; anything else (non-2xx, transport
error) is reported so the caller can log it.
Req is an optional dependency. A host that enables Back-Channel Logout with
the default deliverer must have :req available; one that supplies its own
logout: [http_client: ...] does not.