[C++] Make index kernel work in exec plans

[asfimport]

As written, it gives the wrong result because it's order-dependent. Needs ARROW-13540 for ordering.

Reporter: David Li / @lidavidm

Related issues:

• [C++][Compute] Add OrderByNode for ordering of rows in an ExecPlan (requires)

_{Note: This issue was originally created as ARROW-13591. Please see the migration documentation for further details.}

[github-actions]

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