Add `reason` support on WebSocketDisconnectEvent

[frankie567]

Summary

In the ASGI specification repo, we recently talked about the lack of support for the reason field in the WebSocketDisconnectEvent. This will be likely added to the spec soon.

This PR adds support for this new addition. It doesn't need to be merged before the spec is officially modified, but we're already prepared here 😄

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ ] ~I've updated the documentation accordingly.~ Don't think it's necessary for this change

[frankie567]

Now that the ASGI spec change has been merged, I think we can go forward with this 😄