[C++] Implement sum/mean aggregations over decimals

[asfimport]

For both scalar and hash aggregates.

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

PRs and other links:

• GitHub Pull Request #10931

_{Note: This issue was originally created as ARROW-13613. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 10931
#10931