Again in preparation for handling more than one OIDC provider, add a new caveat to the macaroon used as an OIDC session cookie, which remembers which OIDC provider we are talking to. In future, when we get a callback, we'll need it to make sure we talk to the right IdP.

As part of this, I'm adding an idp_id and idp_name field to the OIDC configuration object. They aren't yet documented, and we'll just use the old values by default.