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ARROW-11981: [C++] Implement Union ExecNode

UnionDataset allows Fragments of multiple schemas and differing file formats to be scanned together as a single Dataset. This is useful functionality but makes the Dataset interface somewhat difficult to reason about since it must be general enough to accommodate UnionDataset.

After ARROW-11928 it will probably be more natural to support unioning of datasets through a subclass of ExecNode. Reconciliation of differing schemas can then be trivially handled by a full ProjectNode.

Note this would obviate both ARROW-11001 and ARROW-11749. In addition, Dataset could be simplified to a concrete class containing a set of compatibly typed/formatted Fragments.

Closes #10927 from aocsa/aocsa/ARROW-11981

Authored-by: Alexander <aocsa.cs@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>