plugin events: update following `Events` protoXEP submission: update the plugin to follow the specification proposed at https://github.com/xsf/xeps/pull/1206 Events are internally converted to a dict following a format described in `event_data_2_event_elt` docstring. RSVP mechanism now uses Pubsub Attachments (XEP-0470), with a user extensible data form. Bridge methods signatures have changed. rel 372

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