Feature: opt-in survey when exiting a game

On first start-up, the game will ask if you want to participate
in our automated survey. You have to opt-in, and can easily opt-out
(via the Options) at any time.

When opt-in, whenever you exit a game, a JSON blob will be send
to the survey server hosted by OpenTTD. This JSON blob contains
information that gives a global picture of the game just played:
- What settings were used
- How many humans vs AIs
- How long the game has been played
- Basic information about the OS / CPU

All this information is kept very generic, so there is no
chance we send private information to our survey server.
Nothing in the JSON blob could identify you as a person; it
mostly tells about the game played. At any time you can see
what the JSON blob includes, by pressing the "Preview Survey
Results" button in-game.

src/network/network_survey.h

@@ -0,0 +1,54 @@
+/*
+ * 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 network_survey.h Part of the network protocol handling opt-in survey. */
+
+#ifndef NETWORK_SURVEY_H
+#define NETWORK_SURVEY_H
+
+#include <condition_variable>
+#include <mutex>
+#include "core/http.h"
+
+/**
+ * Socket handler for the survey connection
+ */
+class NetworkSurveyHandler : public HTTPCallback {
+protected:
+	void OnFailure() override;
+	void OnReceiveData(const char *data, size_t length) override;
+	bool IsCancelled() const override { return false; }
+
+public:
+	enum class Reason {
+		PREVIEW, ///< User is previewing the survey result.
+		LEAVE, ///< User is leaving the game (but not exiting the application).
+		EXIT, ///< User is exiting the application.
+		CRASH, ///< Game crashed.
+	};
+
+	void Transmit(Reason reason, bool blocking = false);
+	std::string CreatePayload(Reason reason, bool for_preview = false);
+
+	constexpr static bool IsSurveyPossible()
+	{
+#ifndef WITH_NLOHMANN_JSON
+		/* Without JSON library, we cannot send a payload; so we disable the survey. */
+		return false;
+#else
+		return true;
+#endif /* WITH_NLOHMANN_JSON */
+	}
+
+private:
+	std::mutex mutex; ///< Mutex for the condition variable.
+	std::condition_variable loaded; ///< Condition variable to wait for the survey to be sent.
+};
+
+extern NetworkSurveyHandler _survey;
+
+#endif /* NETWORK_SURVEY_H */