[C++] Implement more "Datum" types for AggregateKernel

[asfimport]

Currently it gives the following error if the datum isn't an array:

AggregateKernel expects Array datum

Reporter: Philipp Moritz / @pcmoritz

_{Note: This issue was originally created as ARROW-5022. Please see the migration documentation for further details.}

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
I'm not sure we want to make kernels support all datum combination. This will likely be delegated to the query engine.

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
Note that we can probably add "naive" support in the non-kernel but friendly api, e.g. the Sum and Mean functions.

[asfimport]

Wes McKinney / @wesm:
Yes I agree that we should not proliferate kernel-specific public API endpoints and instead work toward a more generic point of entry for query execution. Otherwise we'll end up with a ton of public APIs that may take a lot of energy to maintain

[asfimport]

Antoine Pitrou / @pitrou:
The query engine will have to decide based on the specifics of each kernel, though. Since it's kernel-specific, perhaps it can be implemented in the kernel just as well (at least for the chunked array variant).

[asfimport]

Antoine Pitrou / @pitrou:
@bkietz @lidavidm What do you think the way forward should be here?

[asfimport]

David Li / @lidavidm:
What is left to do here? We added scalar aggregation over scalars in ARROW-9056, and for hash aggregation, I don't think it makes sense to support anything but arrays. So going forward we should just make sure to support scalars and arrays in scalar aggregation.

[asfimport]

Antoine Pitrou / @pitrou:
Indeed, and chunked arrays are supported as well. We can probably close.