[Python] Add custom pickle reducers for DictionaryScalar

[asfimport]

Since there is no sequence converter available for Dictionary type we cannot construct it directly so pa.scalar fail as the reducer function to reconstruct them.

Reporter: Krisztian Szucs / @kszucs
Assignee: Krisztian Szucs / @kszucs

Related issues:

• [C++][Python] Refactor python to arrow conversions based on a reusable conversion API (is fixed by)
• [Python] Support pickling DictionaryScalar (duplicates)

_{Note: This issue was originally created as ARROW-9593. Please see the migration documentation for further details.}