arrow

GH-33206: [C++] Add support for StructArray sorting and nested sort keys (#35727)



### Rationale for this change

We don't currently support sorting `StructArray`s despite already having the high-level facilities to do so. For instance, we allow passing multiple sort keys (based on `FieldRef`s) to sort record batches and tables - but the current implementations are fairly limited since nested refs aren't allowed (due to the burden of null flattening). Since https://github.com/apache/arrow/pull/35197, we now have an easier way to do this.

### What changes are included in this PR?

- Adds support for `StructArray` in `sort_indices`
- Adds support for nested sort keys in `sort_indices` for `RecordBatch`, `ChunkedArray`, and `Table`

### Are these changes tested?

Yes (tests are included)

### Are there any user-facing changes?

Yes
* Closes: #33206

Authored-by: benibus <bpharks@gmx.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>