forward proxy support for client websocket connections

[obno]

• adds forward proxy support to client WS proxy
• adds Dial via HTTP CONNECT tunnel support to TLS ClientHandshake in client side channel credentials

[vikin91]

Thank you for your contribution! I looked briefly at the code, but I couldn't see any tests for the feature. Could you please add integration or unit tests?
I am also not fully following which problem this code is solving. Could you please edit the PR description and describe in details which scenarios were not working for you and how this PR fixes the issue?

[vikin91]

Hi @obno , I wanted to check if you maybe have planned to work further on this PR?

[obno]

Yes. I will work on this towards the end of this week. Setting up unit tests with a 'squid' like forward proxy is a bit involved.

[guyni]

Hi @obno & @vikin91 I also had the same change on client/ws_proxy.go in my fork. In what case is side channel credentials used? I ran the debugger and didn't see side channel credentials thing was called.