[VL] Incorporate SQL Union operator into Velox execution pipeline

Union is currently implemented in Java and the operator could break up one fused Velox's task into two. We'd find a way to offload the operator to Velox so Velox could get a bigger plan for applying more local optimizations.

The issue names the Java Union implementation and the Velox execution pipeline, but no files or tests. Start by locating how Union currently splits fused tasks and how operators are offloaded to Velox; done means Union is incorporated into the Velox plan without breaking task fusion and enables broader local optimization.

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