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/HttpProtocol.cpp

@@ -0,0 +1,24 @@
+#include <LibCore/CHttpJob.h>
+#include <LibCore/CHttpRequest.h>
+#include <ProtocolServer/HttpDownload.h>
+#include <ProtocolServer/HttpProtocol.h>
+
+HttpProtocol::HttpProtocol()
+    : Protocol("http")
+{
+}
+
+HttpProtocol::~HttpProtocol()
+{
+}
+
+RefPtr<Download> HttpProtocol::start_download(PSClientConnection& client, const URL& url)
+{
+    CHttpRequest request;
+    request.set_method(CHttpRequest::Method::GET);
+    request.set_url(url);
+    auto job = request.schedule();
+    if (!job)
+        return nullptr;
+    return HttpDownload::create_with_job({}, client, (CHttpJob&)*job);
+}