[C++] Improve performance on dictionaries for 'case_when' kernel

[asfimport]

#11022 (comment)

If we change the dictionary builders to allow separately encoding a new value (and getting back and index) and appending a dictionary item index, we could avoid re-encoding the same values over and over.

Reporter: David Li / @lidavidm

Related issues:

• [C++] Support dictionaries directly in case_when kernel (relates to)

_{Note: This issue was originally created as ARROW-14042. Please see the migration documentation for further details.}