Support Dictionary Unification with Concatenate

[asfimport]

Seems to be supported through arrow to pandas conversions at the moment. I believe the DictionaryUnifier could be leveraged for this.

Not sure if there are unintended consequences, but the NYI implies that this is desired. Didn't see an open issue for it already.

Reporter: Daniel Nugent / @nugend

Related issues:

• [C++] Implement arrow::Concatenate for dictionary-encoded arrays with unequal dictionaries (duplicates)

_{Note: This issue was originally created as ARROW-9150. Please see the migration documentation for further details.}

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
Is this similar to ARROW-5336 ?

[asfimport]

Daniel Nugent / @nugend:
Yes, looks like. Sorry, I think I searched for "concat" rather than "concatenate" when looking for an existing issue.