[Data] Implement accurate memory accounting for UnionOperator

[bveeramani]

Why are these changes needed?

If an operator in the DAG doesn't implement accurate memory accounting, then the entire pipeline falls back to the legacy (worse) scheduling algorithm. To mitigate this issue, this PR implements memory accounting for UnionOperator.

In addition, this PR:

• Fixes [Data] Dataset.union might error if you use preserve_options #41524
• Simplifies the union tests (which are currently spread across many files)

Related issue number

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• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[bveeramani]

Technically materialize is redundant here because from_items returns a materialzied dataset, but I left it for explictness