[SPARK-3173][SQL] Timestamp support in the parser

If you have a table with TIMESTAMP column, that column can't be used in WHERE clause properly - it is not evaluated properly. [More](https://issues.apache.org/jira/browse/SPARK-3173)

Motivation: http://www.aproint.com/aggregation-with-spark-sql/

- [x] modify SqlParser so it supports casting to TIMESTAMP (workaround for item 2)
- [x] the string literal should be converted into Timestamp if the column is Timestamp.
Space before and after `@`