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/LibHTTP/Job.h

@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/FileStream.h>
 #include <AK/HashMap.h>
 #include <AK/Optional.h>
 #include <LibCore/NetworkJob.h>
@@ -37,7 +38,7 @@ namespace HTTP {
 
 class Job : public Core::NetworkJob {
 public:
-    explicit Job(const HttpRequest&);
+    explicit Job(const HttpRequest&, OutputStream&);
     virtual ~Job() override;
 
     virtual void start() override = 0;
@@ -49,6 +50,7 @@ public:
 protected:
     void finish_up();
     void on_socket_connected();
+    void flush_received_buffers();
     virtual void register_on_ready_to_read(Function<void()>) = 0;
     virtual void register_on_ready_to_write(Function<void()>) = 0;
     virtual bool can_read_line() const = 0;
@@ -56,7 +58,7 @@ protected:
     virtual bool can_read() const = 0;
     virtual ByteBuffer receive(size_t) = 0;
     virtual bool eof() const = 0;
-    virtual bool write(const ByteBuffer&) = 0;
+    virtual bool write(ReadonlyBytes) = 0;
     virtual bool is_established() const = 0;
     virtual bool should_fail_on_empty_payload() const { return true; }
     virtual void read_while_data_available(Function<IterationDecision()> read) { read(); };
@@ -73,11 +75,13 @@ protected:
     State m_state { State::InStatus };
     int m_code { -1 };
     HashMap<String, String, CaseInsensitiveStringTraits> m_headers;
-    Vector<ByteBuffer> m_received_buffers;
+    Vector<ByteBuffer, 2> m_received_buffers;
+    size_t m_buffered_size { 0 };
     size_t m_received_size { 0 };
     bool m_sent_data { 0 };
     Optional<ssize_t> m_current_chunk_remaining_size;
     Optional<size_t> m_current_chunk_total_size;
+    bool m_can_stream_response { true };
 };
 
 }