Now Predicates in Doris can't be pushed down past Aggregation、 Windows and Sort, i will improve it, for three cases.

1. Predicates can be pushed down past Aggregation when the columns the Predicates refer are all the group-by columns.
2. Predicates can be pushwd down past Windows when the columns the Predicates refer are all the partition-by columns.
3. Predicates can be pushwd down past Sort when the query does't include limit and offset.