Implement a sfu server by wrtc in node environment.

pnpm install
node server.js

Then the sfu server will listen on 3004(http), then follow steps below:

1. open ./example/index.html, open console you will see this is user@0.
2. click the publish button, local stream will be capture and render, then send to the sfu server.
3. open another index.html as user@1.
4. clicl the subscribe button, input 0 (the user id of who is publishing stream).
5. then you will see remote stream rendered.

Learned from simple_sfu.

A sfu server shoule be able to build connections via ICE and (un)pack RTP packet which is the real form of a "media stream".

We can do this use lib like libnice but we can also use a complete webrtc implementation. This will save us a lot of work but import many features that we don't need and server ability is restricted by it.

By using wrtc, we can do the two main tasks like:

1. use RTCPeerConnection offer/answer signaling to build ICE connections.
2. pass PC1.tracks to PC2.addTrack in order to sending media stream received and saved in memory to another remote peer.