Use transport ID for authentication for WebTransport connections.

Since token cannot be used twice, we'll use the transport ID for
authentication for WebTransport connections. When WebTransport can be
used for signaling, we don't need another WebSocket connection, then
token can be used here.

Author: jianjunz

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

@@ -28,10 +28,4 @@ It may increase about 2% network cost.
 
 ## Authentication
 
-After creating a WebTransport, a stream with session 0 should be created for authentication. If signaling is 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 following message with UTF-8 encoding in JSON format to server.
-
-```webidl
-interface WebTransportLoginInfo {
-  attribute DOMString? token;  // Base64 encoded token.
-};
-```
+After creating a WebTransport, a stream with session 0 should be created for authentication. If signaling is 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 128 bit length transport ID to the server.