ProtocolServer+LibProtocol: Introduce a server for handling downloads

This patch adds ProtocolServer, a server that handles network requests
on behalf of its clients. The first protocol implemented is HTTP.

The idea here is to use a plug-in architecture where any number of
protocols can be added and implemented without having to mess around
with each client program that wants to use the protocol.

A simple client API is provided through LibProtocol::Client. :^)

Servers/ProtocolServer/Protocol.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <AK/RefPtr.h>
+#include <AK/URL.h>
+
+class Download;
+class PSClientConnection;
+
+class Protocol {
+public:
+    virtual ~Protocol();
+
+    const String& name() const { return m_name; }
+    virtual RefPtr<Download> start_download(PSClientConnection&, const URL&) = 0;
+
+    static Protocol* find_by_name(const String&);
+
+protected:
+    explicit Protocol(const String& name);
+
+private:
+    String m_name;
+};