[C++][Compute] Use generic hash-aggregate for DictionaryArrays

[asfimport]

When calculating unique for chunked DictionaryArrays we currently run through all chunks and unify their dictionaries and then collect chunk indices. We could avoid the dictionary unification by using a generic hash.

See discussion here and here

Reporter: Rok Mihevc / @rok

Related issues:

• [C++] Query engine umbrella issue (is a child of)

_{Note: This issue was originally created as ARROW-12301. Please see the migration documentation for further details.}

[asfimport]

Niranda Perera / @nirandaperera:
@rok do you think this is similar to ARROW-9773?

[asfimport]

Neal Richardson / @nealrichardson:
Anything relating to hashing should be coordinated with the ongoing query engine work (ARROW-12633); cc @michalursa

[asfimport]

Rok Mihevc / @rok:
@nirandaperera Sounds like it could be. Adding both to ARROW-12633.