GH-34698: [C++][Acero] Add node that emits explicit ordering after asserting order

[EnricoMi]

Rationale for this change

An acero node that turns an implicit ordering into an explicit ordering (rows sorted by some columns) is useful to re-use order that already exists in the data.

What changes are included in this PR?

This PR adds the AssertOrderNode that implements this logic. The Scanner employs that node to turn the implicit ordering of the ScanNode into an explicit order as defined by user code via ScanBuilder.Ordering.

Are these changes tested?

There are unit tests for the AssertOrderNode as well as for the ScanNode and ScanBuilder.

Are there any user-facing changes?

The following options has been added:

• the ordering option added to ScanOptions

• the Ordering option added to ScannerBuilder

• the AssertNodeOptions class

• GitHub Issue: #34698

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See also:

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[github-actions[bot]]

:warning: GitHub issue #34698 has been automatically assigned in GitHub to PR creator.

[EnricoMi]

@raulcd thanks, assertions adjusted