Refactoring Windowing for sum() to pass WindowFrameDef instead of two numbers (1 for number of preceding and 1 for number of following)

The functionality should not be affected. Instead of passing 2 numbers (1 for # of preceding rows and 1 for # of following rows), we will pass WindowFrameDef object around. In the following subtasks, it will be used to support additional window like rows between x preceding and y preceding and rows between x following and y following.