[C++] Add function to "conform" a dictionary array to a target new dictionary

[asfimport]

Follow up work to ARROW-554.

Unifying multiple dictionary-encoded arrays is one use case. Another is rewriting a DictionaryArray to be based on another dictionary. For example, this would be used to implement Cast from one dictionary type to another.

This will need to be able to insert nulls where there are values that are not found in the target dictionary

see also discussion at #3165 (comment)

Reporter: Wes McKinney / @wesm

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)
• [C++] Implement kernel function that converts a dense array to dictionary given known dictionary (relates to)

_{Note: This issue was originally created as ARROW-4097. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
This can be implemented as a ScalarFunction I think