[DF] Port Union logic to Datafusion

[jdye64]

Is your feature request related to a problem? Please describe.
Update all existing "Union" logic to replace Apache Calcite data structures with Apache Datafusion data structures.

Describe the solution you'd like
Replace instances of Calcite LogicalUnion with correlating instances of Datafusion Union. This includes any base classes, functions, members that might also rely on that logic.

Describe alternatives you've considered
None

Additional context
Success criteria is for all pytests in test_union.py to be passing

[galipremsagar]

Resolved by #542 & #559