Problem

Currently, Common Table Expression (CTE) is introduced after version 12 and not yet supported in our Postgres system: https://www.postgresql.org/docs/13/queries-with.html
We need to implement the CTE as well as modify the select statement on selecting the CTE

Implementation Plan

1. Create new CTE class in the AST directory
2. Add CTE support in PostgresSelect class
3. Create CTE generation in PostgresExpressionGenerator class
4. Modify the generateSelect() method in PostgresExpressionGenerator class