[C++] Adapt ascii_lower/ascii_upper bulk transforms to work on sliced arrays

[asfimport]

See comments at #7418 (comment)

Also add unit tests to verify that only the referenced data slice has been transformed in the result

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #7458

_{Note: This issue was originally created as ARROW-9122. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 7458
#7458