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.h

@@ -0,0 +1,65 @@
+/*
+ * 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.h Basic functions to send and receive HTTP packets.
+ */
+
+#ifndef NETWORK_CORE_HTTP_H
+#define NETWORK_CORE_HTTP_H
+
+#include "tcp.h"
+
+/** Callback for when the HTTP handler has something to tell us. */
+struct HTTPCallback {
+	/**
+	 * An error has occurred and the connection has been closed.
+	 * @note HTTP socket handler is closed/freed.
+	 */
+	virtual void OnFailure() = 0;
+
+	/**
+	 * We're receiving data.
+	 * @param data   the received data, nullptr when all data has been received.
+	 * @param length the amount of received data, 0 when all data has been received.
+	 * @note When nullptr is sent the HTTP socket handler is closed/freed.
+	 */
+	virtual void OnReceiveData(const char *data, size_t length) = 0;
+
+	/** Silentium */
+	virtual ~HTTPCallback() {}
+};
+
+/** Base socket handler for HTTP traffic. */
+class NetworkHTTPSocketHandler {
+public:
+	/**
+	 * Connect to the given URI.
+	 *
+	 * @param uri      the URI to connect to (https://.../..).
+	 * @param callback the callback to send data back on.
+	 * @param data     optionally, the data we want to send. When set, this will be a POST request, otherwise a GET request.
+	 */
+	static void Connect(const std::string &uri, HTTPCallback *callback, const char *data = nullptr);
+
+	/**
+	 * Do the receiving for all HTTP connections.
+	 */
+	static void HTTPReceive();
+};
+
+/**
+ * Initialize the HTTP socket handler.
+ */
+void NetworkHTTPInitialize();
+
+/**
+ * Uninitialize the HTTP socket handler.
+ */
+void NetworkHTTPUninitialize();
+
+#endif /* NETWORK_CORE_HTTP_H */