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ARROW-13573: [C++] Support dictionaries natively in case_when

This supports dictionaries 'natively', that is, dictionaries are no longer always unpacked. (If mixed dictionary and non-dictionary arguments are given, then they will be unpacked.)

For scalar conditions, the output will have the dictionary of whichever input is selected (or no dictionary if the output is null). For array conditions, we unify the dictionaries as we select elements.

Closes #11022 from lidavidm/arrow-13573

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>