(websocket proxy server) add ability to map different hosts to different websocket servers, using a json config file

ixwebsocket/IXWebSocketProxyServer.h

@@ -7,14 +7,18 @@
 
 #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