[Feature]: Allow route fulfillment with a readable stream body

🚀 Feature Request

Currently, Route.fulfill only accepts string|Buffer as body type. Add nodejs type ReadableStream to permit streamed calls.

Example

const bodyStream = new Readable();

await page.route('**/*', async route => {
 route.fulfill({
    status: 200,
    contentType: 'application/grpc-web+proto',
    body: bodyStream
  });
});

// @todo interact with page

bodyStream.push(chunk);
// @todo  assert on DOM state

bodyStream.push(chunk);
// @todo assert on updated DOM state

bodyStream.destroy();
// @todo assert request completed

Motivation

There are several use-cases where this is useful:

1. stubbing grpc-web calls that are server streaming - the test script could push events into the stream and assert the expected in-progress result is displayed in the webview
2. stubbing server-sent events
3. stubbing streams of other kinds (live video?)

Start at the Route.fulfill and page.route entry points, then trace how the current string|Buffer body is handled. Check how a Node.js Readable stream would flow through fulfillment and verify the grpc-web, server-sent events, and incremental-body examples complete as expected.