Datum is not exported as part of arrow (it is only exported in arrow_array)

[alamb]

Very similar to #4779 but I think this is more serious

Describe the bug
The newly introduced Datum is not exported in arrow even though it is part of the signature of the new compute kernels such as https://docs.rs/arrow/latest/arrow/compute/kernels/cmp/fn.eq.html

This means that to use some of the functions in the arrow crate, a user has to use arrow_array directly, which is inconvenient, or use a deprecated method.

To Reproduce
Search for Datum in arrow: https://docs.rs/arrow/latest/arrow/index.html?search=datum

It only appears in https://docs.rs/arrow-array/46.0.0/arrow_array/trait.Datum.html

Expected behavior
A trait used in the arrow crate API should also be exported in that same crate.

Additional context
Found while updating https://github.com/influxdata/influxdb_iox/pull/8577

[tustvold]

I'm unclear on why this is considered serious, most users won't need to reference the trait, but I do agree we should fix this

[alamb]

I'm unclear on why this is considered serious

I think it is more serious than #4779; I don't think it is particularly serious overall -- more like inconvenient / a rough edge

This means you can't use the new compute kernels for scalars without also importing arrow_array which is annoying

[tustvold]

label_issue.py automatically added labels {'arrow'} from #4791