Add function to handle NTP sync

  Function will be called by both getTime() and getCellularTime() to sync NTP if needed.

Author: pennam

src/ArduinoCellular.cpp

@@ -12,6 +12,17 @@ unsigned long ArduinoCellular::getTime() {
     return Time(year, month, day, hour, minute, second).getUNIXTimestamp();
 }
 
+int ArduinoCellular::syncNTPServer(bool forceNTPSync) {
+    static bool needNTPSync = true;
+    if((needNTPSync || forceNTPSync) && !modem.NTPServerSync()) {
+        // NTP sync failed
+        return 1;
+    }
+    // No sync needed
+    needNTPSync = false;
+    return 0;
+}
+
 ArduinoCellular::ArduinoCellular() {
 }
 
@@ -134,7 +145,7 @@ Time ArduinoCellular::getCellularTime(){
     int minute = 0;
     int second = 0;
     float tz;
-    if (modem.NTPServerSync() == 0) {
+    if (syncNTPServer() == 0) {
       modem.getNetworkTime(&year, &month, &day, &hour, &minute, &second, &tz);
     }
     return Time(year, month, day, hour, minute, second);

src/ArduinoCellular.h

@@ -304,6 +304,8 @@ class ArduinoCellular {
         static unsigned long getTime(); /** Callback for getting the current time as an unix timestamp. */
 
         static constexpr unsigned long waitForNetworkTimeout = 20000L; /**< Maximum wait time for network registration (In milliseconds). */
+
+        static int syncNTPServer(bool forceNTPSync = false); /** Function for synchronizing the NTP server. */
 };