public HttpRequest.Builder streamTo(HttpRequest.Builder request, Method method) {
        InputStream aggregate = data(); // Get the streams now, not when the aggregate is used.
        return request.setHeader("Content-Type", contentType())
                      .method(method.name(), HttpRequest.BodyPublishers.ofInputStream(() -> aggregate));
    }
Streams the aggregate of the current parts of this to the given request builder, and returns it.
Modifications to this streamer after a request builder has been obtained is not reflected in that builder.
This method can be used multiple times, to create new requests.
The request builder's method and content should not be set after it has been obtained.
hosted-api/src/main/java/ai/vespa/hosted/api/MultiPartStreamer.java