feat(05-01): make helper methods internal and add unit tests

- Changed IsThrottleException to internal static in ExecuteQueryRetryHelper
- Changed BuildPagedViewXml to internal static in SharePointPaginationHelper
- Created ExecuteQueryRetryHelperTests: 5 tests (throttle true x3, non-throttle false, nested false)
- Created SharePointPaginationHelperTests: 5 tests (null, empty, whitespace, replace, append)

SharepointToolbox/Core/Helpers/ExecuteQueryRetryHelper.cs

@@ -36,7 +36,7 @@ public static class ExecuteQueryRetryHelper
         }
     }
 
-    private static bool IsThrottleException(Exception ex)
+    internal static bool IsThrottleException(Exception ex)
     {
         var msg = ex.Message;
         return msg.Contains("429") || msg.Contains("503") ||