Add websocket support

[ciarams87]

Proposed changes

Problem: As a user, I want to be able to use NKG to proxy WebSocket traffic, so that I can connect to applications using the WebSocket protocol

Solution: When the "upgrade" header is set, configure it to what is set in the request, and configure the "connection" header to "upgrade". When the "upgrade" header is not set, configure the "connection" header to "close. See https://nginx.org/en/docs/http/websocket.html. This additionally requires that "upgrade" and "connection" headers cannot be overridden using header filters.

Testing: Updated unit-tests, confirmed conformance tests are still passing, and manual testing:
Tested HTTP and WebSocket connections:

WebSocket

* WebSocket connection established
* Preparing request to ws://cafe.example.com/
* Current time is 2023-08-14T15:01:51.443Z
* Using HTTP 1.1

> GET / HTTP/1.1
> host: cafe.example.com:80
> Sec-WebSocket-Version: 13
> Sec-WebSocket-Key: Ul3XldhcMNzcY3c3pvzHcQ==
> Connection: Upgrade
> Upgrade: websocket
> Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits

< HTTP/1.1 101 Switching Protocols
< server: nginx/1.25.1
< date: Mon, 14 Aug 2023 15:01:51 GMT
< connection: upgrade
< upgrade: websocket
< sec-websocket-accept: 1zoiZ45TTpCrRx7HuiFgR/nsCD0=

HTTP connection using http-header-filter example:

1. With upgrade header set

❯ curl -s --resolve echo.example.com:$GW_PORT:$GW_IP http://echo.example.com:$GW_PORT/headers -H "My-Cool-Header:my-client-value" -H "My-Overwrite-Header:dont-see-this" -H "Upgrade:anything"
Headers:
  header 'Accept-Encoding' is 'compress'
  header 'My-cool-header' is 'my-client-value,this-is-an-appended-value'
  header 'My-Overwrite-Header' is 'this-is-the-only-value'
  header 'Host' is 'echo.example.com'
  header 'Upgrade' is 'anything'
  header 'Connection' is 'upgrade'
  header 'Accept' is '*/*'

2. Without upgrade header set

❯ curl -s --resolve echo.example.com:$GW_PORT:$GW_IP http://echo.example.com:$GW_PORT/headers -H "My-Cool-Header:my-client-value" -H "My-Overwrite-Header:dont-see-this"
Headers:
  header 'Accept-Encoding' is 'compress'
  header 'My-cool-header' is 'my-client-value,this-is-an-appended-value'
  header 'My-Overwrite-Header' is 'this-is-the-only-value'
  header 'Host' is 'echo.example.com'
  header 'Connection' is 'close'
  header 'Accept' is '*/*'

Closes #315

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork