[C++] Custom compute functions (Arrow extensibility)

[muriloadriano]

Hello, a couple of questions regarding extensibility in Arrow.

Is there a way to add custom compute functions as an user of Arrow?
Context is that I am evaluating having an extension type that should not be generally relevant for other projects and I wanted to support existing functions (like sort) but also custom functions on existing types that also are probably not of general interest.

It seems to me that the only way to do this would be through changing Arrow internal libraries, or is there a way to do that via public APIs? In the sort example, it seems I could achieve that by providing an [ArrayCountSorter[(https://github.com/apache/arrow/blob/c506b0806bd2b90410400d349a16bc4a5b1dd51c/cpp/src/arrow/compute/kernels/vector_array_sort.cc#L280) for my type but for that I would need to have access to internals like vector_sort_internal.h (so I can't do that without patching Arrow).

Generally, are extension/user types meant to be treated as opaque-only by Arrow? Are there any known efforts on that end that I could follow/contribute to?

Thank you!