[SPARK-3813][SQL] Support "case when" conditional functions in Spark SQL.

"case when" conditional function is already supported in Spark SQL but there is no support in SqlParser. So added parser support to it.

Author : ravipesala ravindra.pesalahuawei.com

Author: ravipesala <ravindra.pesala@huawei.com>

Closes #2678 from ravipesala/SPARK-3813 and squashes the following commits:

70c75a7 [ravipesala] Fixed styles
713ea84 [ravipesala] Updated as per admin comments
709684f [ravipesala] Changed parser to support case when function.