Provide an ExecNode which wraps ScalarAggregateFunctions

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

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

PRs and other links:

• GitHub Pull Request #10705

_{Note: This issue was originally created as ARROW-13313. Please see the migration documentation for further details.}