Add waitForNetworkTimeout constexpr

Author: pennam

Diff for: src/ArduinoCellular.cpp

@@ -217,7 +217,7 @@ bool ArduinoCellular::awaitNetworkRegistration(){
     if(this->debugStream != nullptr){
         this->debugStream->println("Waiting for network registration...");
     }
-    while (!modem.waitForNetwork(20000L)) {
+    while (!modem.waitForNetwork(waitForNetworkTimeout)) {
         if(this->debugStream != nullptr){
             this->debugStream->print(".");
         }

Diff for: src/ArduinoCellular.h

@@ -277,6 +277,8 @@ class ArduinoCellular {
         Stream* debugStream = nullptr; /**< The stream to be used for printing debugging messages. */
 
         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). */
 };