SPARK-3462 push down filters and projections into Unions

Author: Cody Koeninger <cody.koeninger@mediacrossing.com>

Closes #2345 from koeninger/SPARK-3462 and squashes the following commits:

5c8d24d [Cody Koeninger] SPARK-3462 remove now-unused parameter
0788691 [Cody Koeninger] SPARK-3462 add tests, handle compatible schema with different aliases, per marmbrus feedback
ef47b3b [Cody Koeninger] SPARK-3462 push down filters and projections into Unions