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ARROW-10580: [C++] Disallow non-monotonic dense union offsets

The format documentation states that "The respective offsets for each child value array must be in order / increasing."

Closes #9794 from pitrou/ARROW-10580-monotonic-union-offsets

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>