Streaming protocol

[rgbkrk]

Is your feature request related to a problem? Please describe.

I'd love to have a way to push data to the webview in a streaming fashion, whether server sent events or websockets. This got discussed in #420, but is distinctly different as that issue was about asynchronously returning a singular response.

Describe the solution you'd like

Accept Response<Stream<Item = Vec<u8>>> in some manner.

Response (from http) can already accept a Stream

fn stream_response(
    _request: Request<Vec<u8>>,
) -> Result<Response<impl Stream<Item = Vec<u8>> + Send>> {
    let stream = futures::stream::iter(vec![
        "Hello ".as_bytes().to_vec(),
        "World".as_bytes().to_vec(),
    ]);

    Ok(Response::builder()
        .header("Content-Type", "text/plain")
        .status(200)
        .body(stream)
        .unwrap())
}

Where it goes awry is on response via the RequestAsyncResponder, which requires a Response<Into<Cow<'static, [u8]>>>.