Umbrella issue to track implementation of hash aggregate kernels.

Reporter: David Li / @lidavidm

Related issues:

• [C++] String concatenate aggregate kernel (is a parent of)
• [C++] Implement hash_aggregate exact quantile kernel (is a parent of)
• [C++] Implement hash_aggregate mode kernel (is a parent of)
• [C++][Compute] Implement count_distinct/distinct hash aggregate kernels (is a parent of)
• [C++] Implement hash_aggregate mean/stdev/variance kernels (is a parent of)
• [C++] Implement hash_aggregate any/all Boolean kernels (is a parent of)
• [C++][Documentation] List hash aggregate kernels somewhere (is a parent of)
• [C++] Implement hash_aggregate approximate quantile kernel (is a parent of)
• [C++] Add 'count all' option to count (hash) aggregate kernel (is a parent of)
• [C++] Implement product aggregate & hash aggregate kernels (is a parent of)
• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)

_{Note: This issue was originally created as ARROW-13339. Please see the migration documentation for further details.}