[C++] Expose MergeOptions in array Concatenate to unify types

[asfimport]

The arrow::ConcatenateTables function exposes the Field::MergeOptions as a way to indicate how fields with different types should be merged ("unified" / "common type").

The version to concatenate arrays (arrow::Concatenate) currently requires all same-typed arrays. We could add a MergeOptions option here as well?

(this depends on ARROW-14705 to make this option more useful, currently it only handles null -> any upcasts, I think)

Reporter: Joris Van den Bossche / @jorisvandenbossche

_{Note: This issue was originally created as ARROW-15564. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
I think the only tricky thing (same as ConcatenateTables/ARROW-14705) is that we need casts/arrow::compute enabled to support this. ARROW-14705 currently uses an #ifdef but it would be nice to have ARROW-8891 supported.

[github-actions]

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