proxyRequest should be able to proxy streams

[iainsproat]

Describe the feature

Attempting to proxy a request with a stream using proxyRequest currently fails as the body is incorrectly read.

For example, the following will not result in the expected body value of This is a streamed request reaching the /debug path:

app.use(
        "/",
        eventHandler((event) => {
          return proxyRequest(event, url + "/debug", { fetch });
        })
);
      
const stream = new ReadableStream({
        start(controller) {
          controller.enqueue("This ");
          controller.enqueue("is ");
          controller.enqueue("a ");
          controller.enqueue("streamed ");
          controller.enqueue("request.");
          controller.close();
        },
      }).pipeThrough(new TextEncoderStream());

      const result = await fetch(url + "/", {
        method: "POST",
        body: stream,
        headers: {
          "content-type": "application/octet-stream",
          "x-custom": "hello",
        },
        duplex: "half",
      }).then((r) => r.json());

Additional information

• Would you be willing to help implement this feature?

I have provided a rough outline of the problem and potential solution in #374

[pi0]

This is opt-in supported via #413 please check nightly channel in readme if you like to early test.