ProtocolServer: Stream the downloaded data if possible

This patchset makes ProtocolServer stream the downloads to its client
(LibProtocol), and as such changes the download API; a possible
download lifecycle could be as such:
notation = client->server:'>', server->client:'<', pipe activity:'*'
```
> StartDownload(GET, url, headers, {})
< Response(0, fd 8)
* {data, 1024b}
< HeadersBecameAvailable(0, response_headers, 200)
< DownloadProgress(0, 4K, 1024)
* {data, 1024b}
* {data, 1024b}
< DownloadProgress(0, 4K, 2048)
* {data, 1024b}
< DownloadProgress(0, 4K, 1024)
< DownloadFinished(0, true, 4K)
```

Since managing the received file descriptor is a pain, LibProtocol
implements `Download::stream_into(OutputStream)`, which can be used to
stream the download into any given output stream (be it a file, or
memory, or writing stuff with a delay, etc.).
Also, as some of the users of this API require all the downloaded data
upfront, LibProtocol also implements `set_should_buffer_all_input()`,
which causes the download instance to buffer all the data until the
download is complete, and to call the `on_buffered_download_finish`
hook.

Libraries/LibWeb/DOM/XMLHttpRequest.cpp

@@ -92,10 +92,10 @@ void XMLHttpRequest::send()
     // we need to make ResourceLoader give us more detailed updates than just "done" and "error".
     ResourceLoader::the().load(
         m_window->document().complete_url(m_url),
-        [weak_this = make_weak_ptr()](auto& data, auto&) {
+        [weak_this = make_weak_ptr()](auto data, auto&) {
             if (!weak_this)
                 return;
-            const_cast<XMLHttpRequest&>(*weak_this).m_response = data;
+            const_cast<XMLHttpRequest&>(*weak_this).m_response = ByteBuffer::copy(data);
             const_cast<XMLHttpRequest&>(*weak_this).set_ready_state(ReadyState::Done);
             const_cast<XMLHttpRequest&>(*weak_this).dispatch_event(DOM::Event::create(HTML::EventNames::load));
         },