WebSocket Denial Response implementation

[Kludex]

From the ASGI documentation, we have that:

The core ASGI specification does not allow for any control over the denial response, instead specifying that the HTTP status code 403 should be returned, whereas this extension allows an ASGI framework to control the denial response.

This is the behavior that uvicorn has nowadays: it returns 403 instead of allowing customization by the application.

This issue is to track the development of this feature.

The alternative here can also be to not support it, and write a documentation to forward people to hypercorn. That said, I would prefer to implement it on uvicorn.

References:

• WebSocket Denial Response specifications
• Hypercorn implementation of WebSocket Denial Response
• Starlette WebSocket Denial Response support