[FEA] Expose the utilities from `include/cudf/detail/iterators.cuh` in the public API

[mythrocks]

It would be deeply appreciated if the utilities in include/cudf/detail/iterators.cuh could be exposed as part of CUDF's public API.

The following functions are currently used in spark-rapids-jni, with no suitable alternatives available:

1. make_counting_transform_iterator (Easily replaced with a custom function.)
2. make_pair_iterator
3. make_validity_iterator

Duplicating the latter two would necessitate duplication of a fair bit of code: scalar_value_accessor, scalar_optional_accessor, scalar_pair_accessor, etc.

Since these allow the user to assemble custom kernels neatly, and since these behave as extensions to the column interface, it would be good to have them as part of the public API. (cc @davidwendt.)

On a related note, the offsetalator machinery (offsetalator_factory, input_offsetalator, etc.) would also be useful in assembling custom kernels that work on CUDF columns.

[mythrocks]

I currently have a PR up (NVIDIA/spark-rapids-jni#4306) to test out how onerous the duplication of make_pair_iterator etc. might be. This commit is not pretty: NVIDIA/spark-rapids-jni@3d1b068.

I think spark-rapids-jni can reasonably maintain its own copy of make_counting_transform_iterator. But IMHO, the validity / pairwise stuff might be best served from cuDF.