ARROW-12859: [C++] Add ScalarFromJSON for testing

[lidavidm]

ScalarFromJSON(int64(), "null") == Datum(Int64Scalar())
ScalarFromJSON(int64(), "5") == Datum(Int64Scalar(5))

[github-actions]

https://issues.apache.org/jira/browse/ARROW-12859

[bkietz]

This approach might be worthwhile for consistency's sake, but we could also use Scalar::CastTo

[pitrou]

Is there anything left to do here?

[lidavidm]

Is there anything left to do here?

I don't think so unless we really want to use Scalar::CastTo here.

[pitrou]

Can you explain the Scalar::CastTo suggestion? I think I'm missing something.

Also, I notice this isn't actually tested? Perhaps add a test alongside those for ArrayFromJSON. And ensure that a non-1 length array would return an error?

[lidavidm]

For CastTo: I think it was actually the Cast kernel (string->type cast).

I added some basic tests. I don't think there's a good way to hit the DCHECK because that would imply a JSON value converter appended two array values for a single JSON value.

[pitrou]

+1