[C++] Add ScalarFromJSON for easier testing

[asfimport]

We have ArrayFromJSON and various others but not DatumFromJSON which would make things a little easier.

(Originally this suggested making a DatumFromJSON but then it's ambiguous whether something might be, for instance, an array or a list scalar, as Antoine pointed out.)

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

PRs and other links:

• GitHub Pull Request #10386

_{Note: This issue was originally created as ARROW-12859. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
How do you decide whether it's an array or a list scalar, for example?
(or a list array or a chunked array, or a struct array or record batch...)

[asfimport]

David Li / @lidavidm:
Ah, I had only been thinking about primitives…we could either make it PrimitiveDatumFromJSON or just cancel this (tests could also just define an AssertFoo(Datum ...) and then use ScalarFromJSON or ArrayFromJSON as appropriate - would be more verbose but would at least avoid having to define lots of overloads). Or we could define very simple wrappers (DatumFromJSON(int64(), Scalar("[1]")) which could become C++14 user-defined literals (DatumFromJSON(int64(), "[1]"_scalar). (Though, on second thought, the literal syntax doesn't really buy us anything here so there's no point there.)

[asfimport]

David Li / @lidavidm:
Ok, it looks like we don't even have ScalarFromJSON. Let's just add that and defer any other changes - just having that should simplify things somewhat already (e.g. in a type-parameterized test you can just define a scalar(string_view) helper).

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 10386
#10386