arrow

GH-33592: [C++] support casting nullable fields to non-nullable if there are no null values (#43782)

* GitHub Issue: #33592

Notes for myself/fixer:
- [tests that need to get updated](https://github.com/search?q=repo%3Aapache%2Farrow%20%22cannot%20cast%20nullable%20field%22&type=code) (almost definitely not a complete list)
- [update: actually we should handle the go implementation in the go repository.] hmm, looks like [go wrapper does its own nullability checks](https://github.com/apache/arrow/blob/f078942ce2df68de8f48c3b4233132133601ca53/go/arrow/compute/cast.go#L283-L286). I assume this is just an optimization to not have to go into the C++ and hit the error down there. So if we just delete this check then I think the C++ logic will handle all of it???
- [how the plain column handles this cast](https://github.com/apache/arrow/blob/f078942ce2df68de8f48c3b4233132133601ca53/python/pyarrow/table.pxi#L3238-L3243), some logic like this probably needs to get ported over to the struct implementation
- (running from /cpp/build) `cmake .. --preset ninja-debug-basic`, then `cmake --build . && PYTHON=python ctest -R 'arrow-compute-scalar-cast-test' --output-on-failure` to run the specific test
-  to format: `uvx pre-commit run --all-files clang-format`

Lead-authored-by: Nick Crews <nicholas.b.crews@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>