core (xmpp): add `origin` and `origin_label` to message data: If a message comes from a gateway, its type is used as `origin` in `extra`, and it has a name, it's used as `origin_label`. `is-bot` is also set when the sending entity is advertised as a bot. rel 459

these certificates are used for demo purpose only. Those must be the same as in
backend_e2e (they are generated there)