Allow virtual columns to support IN clauses/multiple value filtering

We have customers using virtual columns in an SQL view, e.g: 

Select *, {DECLARE p_postdate date } p_postdate from table where post_date=p_postdate

This functionality needs to be expanded to support multiple values pf p_postdate (i.e, an IN clause, or being able to use combinations of WHERE clauses to filter the value). 

Any enhancement for this should include the following critical success points:

- Allow relational flitering using SQL (e.g, a WHERE clause that can be added to any kind of query, doesn't require being wrapped in a stored procedure, etc.)

- Pushes down all filter values to the underlying data source (does not filter them in memory of TDV)