[C++] Output batch size control in ExecPlan

[westonpace]

Describe the enhancement requested

Currently the exec plan processes things in fairly small (32Ki rows) batches according to the morsel/batch model. However, users may not want batches that small to be returned from Acero.

It shouldn't be too difficult to add an option to specify an output batch size and then, in the sink, to merge batches into larger output batches.

At the moment that would introduce a memory copy. In the longer term, we could investigate approaches that avoid this memory copy, but that could be addressed beyond this issue.

Component(s)

C++