add NTP time synchronization module

src/utun_instance.h

@@ -20,6 +20,7 @@ extern "C" {
 #include "proxy/tcp_proxy_client.h"
 #include "etcp_router.h"
 #include "proxy/tcp_proxy_server.h"
+#include "ntp_time.h"
 
 #include "etcp_api.h"
 #include "config_parser.h"
@@ -145,6 +146,9 @@ struct UTUN_INSTANCE {
     // Pending background connections (etcp_connect API)
     struct ETCP_CONNECT* pending_connects;
     uint32_t etcp_connect_timeout_tb;  // Initial timeout in 0.1ms units, default 20000 (2s)
+
+    // NTP time synchronization
+    struct NTP_TIME ntp;
 };
 
 // Functions