Authorizes without a person: GETs the authorization URL, does not follow
the redirect it answers with, and reads the authorization response from
its Location header.
Sake.Client.connect(url, auth: [authorize: &Sake.Client.Auth.Headless.authorize/1])Only for an authorization server that approves at once — a test server,
the conformance suite's, or one that recognizes the caller from
something passed in req: (a session cookie, say) — never for one that
shows a login page or asks for consent.