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ARROW-14027: [C++] Handle scalars in Grouper

This adds basic support for scalar values in the groupers. The non-fast grouper directly supports scalar. GrouperFastImpl will expand scalars to arrays first; this is because the rest of the implementation deals with buffers, which we don't really have with a scalar, so expanding scalars was the most expedient solution at a cost in performance. There's also a bit of refactoring to consistently expose a view method for primitive scalars, which can later be used to simplify some kernel implementations.

Closes #11191 from lidavidm/arrow-14027

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>