Add ModifyWebSocketResponsePlugin

[abhinavsingh]

As a starting point for working with web sockets.

• Fixes [WebSocketFrame] Exception when building frames with very large payloads #912
• Background Modify websocket messages #913

[codecov]

Codecov Report

Attention: Patch coverage is 61.70213% with 18 lines in your changes missing coverage. Please review.

Project coverage is 87.64%. Comparing base (d4449b8) to head (8ac4cac).
Report is 270 commits behind head on develop.

Files	Patch %	Lines
proxy/http/websocket/frame.py	66.66%	6 Missing and 5 partials ⚠️
proxy/plugin/modify_websocket_response.py	45.45%	6 Missing ⚠️
proxy/plugin/web_server_route.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #914      +/-   ##
===========================================
- Coverage    87.80%   87.64%   -0.16%     
===========================================
  Files          131      132       +1     
  Lines         5903     5917      +14     
  Branches       590      593       +3     
===========================================
+ Hits          5183     5186       +3     
- Misses         619      631      +12     
+ Partials       101      100       -1     

Flag	Coverage Δ
pytest	87.47% <61.70%> (-0.16%)	⬇️

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