How to use fetch in workers rs

If there was the following code in javascript as a response,

```
return await fetch("http://websocket.website.com", { headers: { "Upgrade": "websocket" } })
```

How would you do the same with workers-rs, to proxy a websocket request.

The issue provides a JavaScript fetch example and asks how to proxy a WebSocket request with workers-rs, but names no files or tests. Start by locating the workers-rs fetch and WebSocket entry points, then verify an equivalent request flow; done means the supported approach is demonstrated and works for the stated proxy case.

Written by the indexing model from the issue text.