[WIP] Implement a simple TCPProxy filter

[nevill]

Yes, I see @jxd134 's PR #234 there, but I cannot really run the code there (got a panic when starting server).

This PR is a quick concept proof as discussed in #229 (comment). I think we can have further discussion based on a working code base.

To play with filter TCPPorxy, you have to config with it, like

---
kind: HTTPServer
name: server-demo
port: 10080
keepAlive: true
https: false
rules:
  - paths:
      - pathPrefix: /tcp
        backend: tcp-demo
---
kind: HTTPPipeline
name: tcp-demo
flow:
  - filter: proxy
filters:
  - name: proxy
    kind: TCPProxy
    servers:
      - url: tcp://127.0.0.1:9095
      - url: tcp://127.0.0.1:9096
      - url: tcp://127.0.0.1:9097

Then you can go run example/backend-service/mirror/mirror.go to start the back-end servers, try to curl it, like

curl -v example.com:10080/tcp
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to example.com (127.0.0.1) port 10080 (#0)
> GET /tcp HTTP/1.1
> Host: example.com:10080
> User-Agent: curl/7.64.1
> Accept: */*
> 
< HTTP/1.1 200 OK
< Content-Length: 170
< Content-Type: text/plain; charset=utf-8
< Date: Tue, 14 Sep 2021 02:36:32 GMT
< 
Your Request
==============
Method: GET
Host: example.com:10080
URL   : /tcp
Header:
    Accept: [*/*]
    Accept-Encoding: [gzip]
    User-Agent: [curl/7.64.1]
Body  : 
* Connection #0 to host example.com left intact
* Closing connection 0

[codecov-commenter]

Codecov Report

Merging #255 (d478cfc) into main (a915688) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #255   +/-   ##
=======================================
  Coverage   80.12%   80.12%           
=======================================
  Files          44       44           
  Lines        4835     4835           
=======================================
  Hits         3874     3874           
  Misses        740      740           
  Partials      221      221           

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[jxd134]

@nevill my tcp proxy code is incomplete.

[megaeasex]

[TASK:easegress-pr-test FAILED]megaease/easegress Pull Request 255 Deploy Test failed

[nevill]

Since #234 is ready to review, closing this one.