Refactor streaming api code to a factory or a common function

[jsartisan]

Right now, we are using creating streaming request directly using Request and fetch directly like this:

const request = new Request(apiURL, {
    method: "POST",
    headers: headers,
    body: JSON.stringify(requestBody),
  });

  return fetch(request);

We should have a common function that adds all the required headers for streaming and uses them in all places.