Implement NetworkManager

For now there are two methods:

- `NETWORK:IsUrlAllowed()`: Check whether access to a certain URL is allowed.
- `NETWORK:HttpRequest()`: Perform an HTTP request.

By default access to the network is disabled for all target hosts. It
can be enabled by setting `HttpEnable=1` in the preferences. Individual
hosts have to be added to `HttpAllowHosts` as a comma separated list to
allow access.

See included docs for more details on usage.

extern/IXWebSocket-11.3.2/ixwebsocket/IXWebSocketProxyServer.h

@@ -0,0 +1,24 @@
+/*
+ *  IXWebSocketProxyServer.h
+ *  Author: Benjamin Sergeant
+ *  Copyright (c) 2019-2020 Machine Zone, Inc. All rights reserved.
+ */
+#pragma once
+
+#include "IXSocketTLSOptions.h"
+#include <cstdint>
+#include <map>
+#include <stddef.h>
+#include <string>
+
+namespace ix
+{
+    using RemoteUrlsMapping = std::map<std::string, std::string>;
+
+    int websocket_proxy_server_main(int port,
+                                    const std::string& hostname,
+                                    const ix::SocketTLSOptions& tlsOptions,
+                                    const std::string& remoteUrl,
+                                    const RemoteUrlsMapping& remoteUrlsMapping,
+                                    bool verbose);
+} // namespace ix