plugin XEP-0166: `jingle_terminate`, session handling and reason parsing: - a new `jingle_terminate` bridge method let frontend terminate a session - methods to manage a session from another plugin - methods to parse `<reason>` element from another plugin - allow session to be created before a first jingle request is received, to prepare for `preflight` methods rel 423

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