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GH-38381: [C++][Acero] Create a  sorted merge node (#38380)



### Rationale for this change

This is an implementation of a node that can merge N sorted inputs (only in ascending order for a first pass).

Where possible I have shared components with `asof_join_node.cc`.

Full description/use case is described in https://github.com/apache/arrow/issues/38381

### What changes are included in this PR?

* Take out relevant guts of asofjoin to stream data top to bottom/consume in a non blocking manner
* Implement a sorted merger

### Are these changes tested?

Basic test added. Locally I have tested this on 100+ gigabytes of parquet, sharded across 50+ files. Happy to add a benchmark test on top of the basic test, but submitting now for code feedback.

### Are there any user-facing changes?

Yes, `sorted_merge` is now an exposed declaration

Lead-authored-by: Jeremy Aguilon <jeraguilon@gmail.com>
Co-authored-by: jeremy <jeraguilon@gmail.com>
Co-authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>