Add: use https:// for content-service connections (#10448)

This requires the use of WinHTTP (for Windows) or libcurl (for all
others except Emscripten). Emscripten does not support http(s)
calls currently.

On Linux it requires ca-certificates to be installed, so the HTTPS
certificate can be validated. It is really likely this is installed
on any modern machine, as most connections these days are HTTPS.

(On MacOS and Windows the certificate store is filled by default)

Reminder: in case the http(s):// connection cannot be established,
OpenTTD falls back to a custom TCP-based connection to fetch the
content from the content-service. Emscripten will always do this.

src/network/core/http_none.cpp

@@ -0,0 +1,37 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/**
+ * @file http_emscripten.cpp Emscripten-based implementation for HTTP requests.
+ */
+
+#include "../../stdafx.h"
+#include "../../debug.h"
+#include "../../rev.h"
+#include "../network_internal.h"
+
+#include "http.h"
+
+#include "../../safeguards.h"
+
+/* static */ void NetworkHTTPSocketHandler::Connect(const std::string &uri, HTTPCallback *callback, const char *data)
+{
+	/* No valid HTTP backend was compiled in, so we fail all HTTP requests. */
+	callback->OnFailure();
+}
+
+/* static */ void NetworkHTTPSocketHandler::HTTPReceive()
+{
+}
+
+void NetworkHTTPInitialize()
+{
+}
+
+void NetworkHTTPUninitialize()
+{
+}