[C++][Python] Implement HashAggregate UDF

[asfimport]

Implementing hash-aggregate user-defined functions and use these functions with group_by/ agg operations.

Reporter: Vibhatha Lakmal Abeykoon / @vibhatha
Assignee: Vibhatha Lakmal Abeykoon / @vibhatha

_{Note: This issue was originally created as ARROW-18279. Please see the migration documentation for further details.}