ARROW-11928 adds ExecNodes, to which GroupByNode should be added so that a dataset scan can terminate in a grouped aggregation

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

Related issues:

• [C++] Query engine umbrella issue (is a child of)
• [C++][Compute] Support merging GroupByState for multithreaded aggregation (fixes)
• [C++] Implement Hash Aggregation query execution node (is depended upon by)

PRs and other links:

• GitHub Pull Request #10660

_{Note: This issue was originally created as ARROW-12759. Please see the migration documentation for further details.}