UDAF with DISTINCT

Currently, the DISTINCT keyword only works with COUNT, SUM, AVG, MIN, and MAX. With MIN and MAX, it means nothing. Internally, the difference in processing, both in the Server portion and the Columnstore Portion of processing is completely separate from non-DISTINCT.

This ticket is to add DISTINCT capabilities to the Columnstore code, allowing specific UDAF to be created as DISTINCT, even though the parser won't pass the DISTINCT keyword.

A separate ticket is created for UDAnF Window Functions