[C++][Compute] Implement count_distinct/distinct hash aggregate kernels

[asfimport]

Implement count distinct aggregate reusing hash table from hash group by inside of it.

This brings support to SQL queries like:
select a, count(distinct b), count(distinct c) from t group by a

For instance to compute count(distinct b), the first group id mapping will give group id based on column a value; then the second group id mapping is done using the key (groupid(a), b) inside count(distinct b) aggregate (similarly for count(distinct c)). 
After all input rows are consumed, the final processing step scans the hash tables based on (groupid(a), b) and updates an array of counts indexed by groupid(a).
The resulting array of counts represents the output of count distinct aggregate.

Reporter: Michal Nowakiewicz / @michalursa
Assignee: David Li / @lidavidm
Watchers: Rok Mihevc / @rok

Related issues:

• [C++] Query engine umbrella issue (is a child of)
• [C++] Implement hash_aggregate kernels (umbrella issue) (is a child of)
• [C++][Compute] Implement non-hash count_distinct aggregate kernel (is related to)
• [R] Binding for n_distinct() (is depended upon by)

PRs and other links:

• GitHub Pull Request #10876

_{Note: This issue was originally created as ARROW-12728. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
If I understand right, this should be a fairly straightfoward hash aggregate kernel on top of ARROW-12759 - is anyone planning on working on this? I'd like to take it otherwise to get more familiar with the ExecNode/Grouper infrastructure.

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 10876
#10876