[feature] WebSocket support

[benja-wu]

Background

• Easegress is popular for reverse proxy usage in production and development scenarios.
• In reversed proxy use cases, WebSocket is a widely used protocol for a full-duplex communication solution between client and server. It relies on TCP.[1]
• Many reverse proxy support WebSocket,e.g., NGINX[2], Traefik, and so on.

Proposal

• Bring WebSocketServer as an BusinessController to Easegress. (Should treat it as an HTTPServer like traffic gate in the future. )
• Using github.com/gorilla/websocket as the WebSocket client implementation, since it has rich features supproted and a quite active community.（15k star/2.5k fork) Comparing to the original golang.org/x/net/websocket package, it can receive fragmented message and send close message.[3]

Design

1. Spec

kind: WebSocketServer
name: websocketSvr
https: false                  # client need to use http/https firstly for connection upgrade      
certBase64:
keyBase64:
port: 10020                   # proxy servers listening port

backend: ws://localhost:3001  # the reserved proxy target
                              #  Easegress will exame the backend URL's scheme, If it starts with `wss`,
                              #  then `wssCerBase64` and `wssKeyBase64` must not be empty

wssCertBase64:                # wss backend certificate in base64 format
wssKeyBase64:                 # wss backend key in base64 format

2. Request sequence

   +--------------+                +--------------+                +--------------+  
   |              |  1             |              |   2            |              | 
   |   client     +--------------->|  Easegress   +--------------->|  websocket   |
   |              |<---------------+(WebSocketSvr)|<---------------+  backend     | 
   |              |  4             |              |   3            |              |
   +--------------+                +--------------+                +--------------+

3. The WebSocketServer shares some configurations with HTTPServer. Since WebSocket runs on HTTP 1.1 only, and it works on opening two long-life goroutines for the reverse proxy scenario, it's quite different from the original Easegress' HTTPServer+Pipeline's process. So I decide not to implement it into HTTPServer and catalog it into TrafficGate in the first version right now.

4. I will implement this WebSocketServer first version and try to figure out how to integrate with the original HTTPServer at the same time. (Moreover, multiple protocols supporting requirements, such as MQTT, TPC, are on the way... )

References

1. https://datatracker.ietf.org/doc/html/rfc6455
2. https://www.nginx.com/blog/websocket-nginx/
3. https://github.com/gorilla/websocket