[C++] Add options to ValueCount/Unique/DictEncode kernel to toggle null behavior

[asfimport]

Reporter: Francois Saint-Jacques / @fsaintjacques

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)

_{Note: This issue was originally created as ARROW-5530. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
a HashOptions would also need to be introduced

[github-actions]

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