[C++] Add ChunkedArray::GetScalar

[asfimport]

Now that we have Array::GetScalar implemented for most datatypes, adding ChunkedArray::GetScalar would be a natural improvement.

Reporter: Antoine Pitrou / @pitrou

Related issues:

• [C++] Add nulls position option to sort functions (is superceded by)

_{Note: This issue was originally created as ARROW-13545. Please see the migration documentation for further details.}

[asfimport]

Weston Pace / @westonpace:
@pitrou I think this was done as part of ARROW-12063 (https://github.com/apache/arrow/blame/master/cpp/src/arrow/chunked_array.cc#L148) . Can we close this?

[asfimport]

Antoine Pitrou / @pitrou:
Thanks for the notice :)