[Data] Add configuration to PhysicalOperator to allow disabling operator fusion

[bveeramani]

Why are these changes needed?

Ray Data automatically fuses task pool operators together. However, in some cases, we want to not fuse task pool operators. To allow us to disable fusion in those cases, this PR adds a supports_fusion parameter to the physical MapOperator.

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[raulchen]

The default should be False. Only map supports fusion