[VL] Incorporate SQL Union operator into Velox execution pipeline

[zhztheplayer]

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.

[FelixYBW]

Have we observed any performance issue of the break?

[zhztheplayer]

Have we observed any performance issue of the break?

There is no performance issue so far. The current implementation doesn't involve in any data copy yet.

[FelixYBW]

We may lower its priority if so.

[zhztheplayer]

I had a PoC at #7842. Union is comparatively special as it requires multiple Velox execution pipelines. Implementing it should help us on understanding and improving Velox's single threaded execution routine.