Ack content session ID for signaling stream. (#868)

* Ack content session ID for signaling stream.

It helps client to know when authentication for QUIC connection is
finished.

* Update doc to QUIC authentication messages.

Since the receive side acks the same content session ID to sender, and
the content session ID for signaling stream is 0, signaling stream should
ack 0s to client.

Author: jianjunz

doc/design/quic-transport-payload-format.md

@@ -41,4 +41,4 @@ After creating a WebTransport, a stream with session 0 should be created for aut
 
 ## Authentication
 
-If signaling messages are transmitted over WebTransport, authentication follows the regular process defined by [Client-Portal Protocol](https://github.com/open-webrtc-toolkit/owt-server/blob/master/doc/Client-Portal%20Protocol.md). Otherwise, client sends a token for WebTransport as a signaling message. WebTransport token is issued during joining a conference. If token is valid, server sends a 128 bit length transport ID to client. The transport ID should be same as the one included in token.
+If signaling messages are transmitted over WebTransport, authentication follows the regular process defined by [Client-Portal Protocol](https://github.com/open-webrtc-toolkit/owt-server/blob/master/doc/Client-Portal%20Protocol.md). Otherwise, client sends a token for WebTransport as a signaling message. WebTransport token is issued during joining a conference. If the token is valid, server sends a 128 bit length zeros to client.

source/agent/quic/webtransport/quicTransportServer.js

@@ -126,6 +126,8 @@ module.exports = class QuicTransportServer extends EventEmitter {
                   stream.transportId = token.transportId;
                   this._connections.set(connection.transportId, connection);
                   this.emit('connectionadded', connection);
+                  const uuidBytes = this._uuidStringToUint8Array(zeroUuid);
+                  stream.write(uuidBytes, uuidBytes.length);
                 });
                 return;
               } else {  // Store the data.