Http[s]Download: Make the constructor's initialization DRY

Problem:
- `HttpDownload()` and `HttpsDownload()` implementations are the same
  except for types and certificates.

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

Userland/Services/ProtocolServer/HttpCommon.h

@@ -28,14 +28,50 @@
 
 #include <AK/ByteBuffer.h>
 #include <AK/HashMap.h>
+#include <AK/NonnullOwnPtr.h>
+#include <AK/Optional.h>
 #include <AK/OwnPtr.h>
 #include <AK/String.h>
+#include <AK/Types.h>
 #include <LibHTTP/HttpRequest.h>
 #include <ProtocolServer/ClientConnection.h>
 #include <ProtocolServer/Download.h>
 
 namespace ProtocolServer::Detail {
 
+template<typename TSelf, typename TJob>
+void init(TSelf* self, TJob job)
+{
+    job->on_headers_received = [&](auto& headers, auto response_code) {
+        if (response_code.has_value())
+            self->set_status_code(response_code.value());
+        self->set_response_headers(headers);
+    };
+
+    job->on_finish = [&](bool success) {
+        if (auto* response = job->response()) {
+            self->set_status_code(response->code());
+            self->set_response_headers(response->headers());
+            self->set_downloaded_size(self->output_stream().size());
+        }
+
+        // if we didn't know the total size, pretend that the download finished successfully
+        // and set the total size to the downloaded size
+        if (!self->total_size().has_value())
+            self->did_progress(self->downloaded_size(), self->downloaded_size());
+
+        self->did_finish(success);
+    };
+    job->on_progress = [&](Optional<u32> total, u32 current) {
+        self->did_progress(total, current);
+    };
+    if constexpr (requires { job->on_certificate_requested; }) {
+        job->on_certificate_requested = [&](auto&) {
+            self->did_request_certificates();
+        };
+    }
+}
+
 template<typename TBadgedProtocol, typename TPipeResult>
 OwnPtr<Download> start_download(TBadgedProtocol&& protocol, ClientConnection& client, const String& method, const URL& url, const HashMap<String, String>& headers, ReadonlyBytes body, TPipeResult&& pipe_result)
 {