Add websocket support with http-proxy

[virusvfv]

I removed previous PR and separated it into 2 part. This PR is about websocket and proxy support. It does not depends of cloudflare GO forks. So it can be pulled without any problems. In this PR:

• support websocket transmission by providing https:// prefix to server. (ex: angent.exe -connect https://c2-server.com:8443 )
• support http-proxy server with auth in websocket mode (ex: angent.exe -connect https://c2-server.com:8443 -proxy http://admin:petuh@172.16..88:8080 )
• additionally agent session checks in proxy (if agent goes away checker will delete its session from server. So if server has many agents session command will be more informational.

ECH functions I moved into separate brunch in my fork (ws_proxy_ECH). If (or when) GO will support for ECH it can be pulled into master brunch. Or you can also create separate brunch with ECH support as in my fork ). I think it will be useful...

[nicocha30]

Hey @virusvfv , thanks for the PR, i added some remarks that should be addressed before merging