Requires ARROW-12759.

We have a scalar aggregate kernel for this already and hopefully the implementation can be reused.

Also create a pair of kernels median() and hash_median() that wrap this for convenient use by bindings (like in ARROW-14052).

Reporter: David Li / @lidavidm

Related issues:

• [C++] Implement hash_aggregate kernels (umbrella issue) (is a child of)
• [C++] Implement hash_aggregate approximate quantile kernel (is related to)
• [R] Bind median() and quantile() to exact not approximate median and quantile (is depended upon by)

_{Note: This issue was originally created as ARROW-13309. Please see the migration documentation for further details.}