add: exercise answer for chapter 6 and 7

exercises/6/main.http.cpp

@@ -0,0 +1,19 @@
+//
+// main_http.cpp
+// web_server
+// created by changkun at changkun.de/modern-cpp
+//
+
+#include <iostream>
+#include "server.http.hpp"
+#include "handler.hpp"
+
+using namespace LabexWeb;
+
+int main() {
+    // HTTP server runs in port 12345 HTTP, enable 4 threads
+    Server<HTTP> server(12345, 4);
+    std::cout << "Server starting at port: 12345" << std::endl;
+    start_server<Server<HTTP>>(server);
+    return 0;
+}