Avoid forced copy in `Array::logical_nulls`

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
Some code requires "logical nullability" which is distinct from the "physical" nullability (what is defined by an Arrow arrays NullBuffer, if it has one)

To this end, arrow includes `Array::logical_nulls` which will compute logical nulls.

The issue is that it also potentially copies the `NullBuffer` for arrays that have it pre-computed, such as `PrimitiveArray` and `StringArray`. While the copy is perhaps not that bad (it is several `Arc`s) I would like to avoid it entirely if possible

**Describe the solution you'd like**
I would like to change `logical_nulls` to return a `Cow` to avoid the copy, as I propose in https://github.com/coralogix/arrow-datafusion/pull/221

**Describe alternatives you've considered**
We can not make any changes

**Additional context**
Thus came up in DataFusion PR https://github.com/apache/arrow-datafusion/pull/8511

Logical nulls was added in https://github.com/apache/arrow-rs/pull/4691

Start at Array::logical_nulls and review the proposed Cow approach in coralogix/arrow-datafusion#221, along with the DataFusion context in PR #8511. Trace the existing callers and API implications; the work is done when precomputed null buffers avoid the forced copy while logical nulls still behave correctly for other arrays.

Written by the indexing model from the issue text.