[C++][Compute] Improve memory efficiency in Grouper

[asfimport]

There are APIs in arrow::compute::Grouper (GetUniques, Consume) which may be able to be refactored to write into preallocated memory or otherwise have a mode that does less mandatory allocation. We can investigate at some point

Reporter: Wes McKinney / @wesm

Related issues:

• [C++] Improve array expression and kernel evaluation performance on small inputs (relates to)

_{Note: This issue was originally created as ARROW-17129. Please see the migration documentation for further details.}