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/LibCore/NetworkJob.h

@@ -27,6 +27,7 @@
 #pragma once
 
 #include <AK/Function.h>
+#include <AK/Stream.h>
 #include <LibCore/Object.h>
 
 namespace Core {
@@ -43,6 +44,8 @@ public:
     };
     virtual ~NetworkJob() override;
 
+    // Could fire twice, after Headers and after Trailers!
+    Function<void(const HashMap<String, String, CaseInsensitiveStringTraits>& response_headers, Optional<u32> response_code)> on_headers_received;
     Function<void(bool success)> on_finish;
     Function<void(Optional<u32>, u32)> on_progress;
 
@@ -62,13 +65,16 @@ public:
     }
 
 protected:
-    NetworkJob();
+    NetworkJob(OutputStream&);
     void did_finish(NonnullRefPtr<NetworkResponse>&&);
     void did_fail(Error);
     void did_progress(Optional<u32> total_size, u32 downloaded);
 
+    size_t do_write(ReadonlyBytes bytes) { return m_output_stream.write(bytes); }
+
 private:
     RefPtr<NetworkResponse> m_response;
+    OutputStream& m_output_stream;
     Error m_error { Error::None };
 };