Http[s]Protocol: Make the code start_download DRY

Problem:
- `HttpProtocol::start_download` and `HttpsProtocol::start_download`
  implementations are the same except for a few types.

Solution:
- Follow the "Don't Repeat Yourself" mantra and de-duplicate the code
  using templates.

Userland/Services/ProtocolServer/HttpProtocol.h

@@ -26,14 +26,26 @@
 
 #pragma once
 
+#include <AK/ByteBuffer.h>
+#include <AK/HashMap.h>
+#include <AK/OwnPtr.h>
+#include <AK/String.h>
+#include <AK/URL.h>
+#include <LibHTTP/HttpJob.h>
+#include <ProtocolServer/ClientConnection.h>
+#include <ProtocolServer/Download.h>
+#include <ProtocolServer/HttpDownload.h>
 #include <ProtocolServer/Protocol.h>
 
 namespace ProtocolServer {
 
 class HttpProtocol final : public Protocol {
 public:
+    using JobType = HTTP::HttpJob;
+    using DownloadType = HttpDownload;
+
     HttpProtocol();
-    virtual ~HttpProtocol() override;
+    ~HttpProtocol() override = default;
 
     virtual OwnPtr<Download> start_download(ClientConnection&, const String& method, const URL&, const HashMap<String, String>& headers, ReadonlyBytes body) override;
 };