Allow user to use use their own PivotTableHelper implementation

public class XSSFPivotTableHelpers {
    public static final XSSFPivotTableHelpers instance = new XSSFPivotTableHelpers(); 
    private PivotTableHelper _helper;
    
    // add this api for user to replace PivotTableHelper
    public static void initHelper(PivotTableHelper helper){
        instance._helper = helper; 
    }
}