Suggestion: Introduce support for GROUP BY clause in SQL queries.

[arjun12102019]

Added new API to support Group By clause in SQL queries.

Introduced Rest API "/get_group_by_sql" which will append a Group By clause to existing queries.

Please review the changes:

https://github.com/chat2db/Chat2DB/pull/1366/commits/f6315afeb6b929ac3149f82649dbcae4e8008c73.

To-do: Need to override the new method introduced in RdbWebConverter ->" GroupByParam request2param(GroupByRequest request)" I cannot find the implementation class ("RdbWebConverterImpl") of the above interface in the source code. please help me here