New Feature: Open a web socket by providing a jwt token for authentication

Hello everybody,
First of all I would like to say a big thank you to all of this repo developers out there.
Thank you for offering this repo to the open source community.

Having said that, I have used websocket-sharp myself for opening web socket connections from web clients to .NET console applications. While developing, the need arose for a web socket server that can authenticate users trying to open the websocket by providing a jwt token in the url e.g. wss://address/path?token=jwtToken. From what I can tell, the current implementation does not support such kind of authentication so I implemented it myself. I added an event that gets raised during the ws upgrade request, containing the token. Then, the event listener can get the token from the event, validate it and either proceed with the upgrade or close the connection.

You can find the relevant commit here:
https://github.com/kazepis/websocket-sharp/commit/0060bdb097f7af8650699692bf60c45d248ba6b6
I also upgraded everything to .NET 4.8 for MY needs. The massive diffs were produced because I have different formatting settings.
You can easily find the actual change by searching for the event public event EventHandler<HttpRequestEventArgs> OnWebSocketUpgradeRequest;

If this functionality addition is welcome in this repo I would happily make a "clean" pull request.
If yes, are there any contributing guidelines I should follow? So please let me know.

Best,
N.

Start by reviewing commit 0060bdb097f7af8650699692bf60c45d248ba6b6 and search the repository for OnWebSocketUpgradeRequest. Trace the WebSocket upgrade request flow and compare the proposed event behavior with the existing server API. Done means the project has an agreed JWT-token authentication extension with suitable validation and connection-handling tests.