streaming: add support for RTSP mountpoints and events

[lminiero]

The Streaming plugin in Janode didn't support creating RTSP mountpoints yet. Considering we recently added support for notifying when RTSP mountpoints get disconnected/reconnected in Janus (see meetecho/janus-gateway#3578 for details), I decided to kill two birds with one stone. (PS: as a side note, we may have to add support for creating live/ondemand mountpoints too, even though no-one really uses those).

Example of creating an RTSP mountpoint:

await janodeManagerHandle.createRtpMountpoint({ id: 99, name: 'RTSP from Janode',
    video: { fmtp: 'profile-level-id=42e01f;packetization-mode=1' },
    rtsp: { url: 'rtsp://127.0.0.1:9554/test', notifyChanges: true } });

Being notified about reconnections and disconnections as a subscriber:

streamingHandle.on(StreamingPlugin.EVENT.STREAMING_RTSP_DISCONNECTED, evtdata => {
	Logger.info(`${LOG_NS} ${streamingHandle.name} RTSP disconnected ${JSON.stringify(evtdata)}`);
});
streamingHandle.on(StreamingPlugin.EVENT.STREAMING_RTSP_RECONNECTED, evtdata => {
	Logger.info(`${LOG_NS} ${streamingHandle.name} RTSP connected ${JSON.stringify(evtdata)}`);
});