[C++][Compute] Prototype version of hash aggregation

[asfimport]

Reporter: Neal Richardson / @nealrichardson
Assignee: Michal Nowakiewicz / @michalursa

PRs and other links:

• GitHub Pull Request #9621

_{Note: This issue was originally created as ARROW-11591. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 9621
#9621