ARROW-11591 adds support for grouped aggregation with a very basic hash table.

Reporter: Ben Kietzman / @bkietz
Assignee: Michal Nowakiewicz / @michalursa

Related issues:

• [C++] Query engine umbrella issue (is a child of)

PRs and other links:

• GitHub Pull Request #10290
• GitHub Pull Request #9768

_{Note: This issue was originally created as ARROW-12010. Please see the migration documentation for further details.}