arrow

GH-36892: [C++] Fix performance regressions in `FieldPath::Get` (#37032)



### Rationale for this change

https://github.com/apache/arrow/pull/35197 appears to have introduced significant performance regressions in `FieldPath::Get` - indicated [here](https://conbench.ursa.dev/compare/runs/9cf73ac83f0a44179e6538b2c1c7babd...3d76cb5ffb8849bf8c3ea9b32d08b3b7/), in a benchmark that uses a wide (10K column) dataframe.

### What changes are included in this PR?

- Adds basic benchmarks for `FieldPath::Get` across various input types, as they didn't previously exist
- Addresses several performance issues. These came in the form of extremely high upfront costs for the `RecordBatch` and `ArrayData` overloads specifically
- Some minor refactoring of `NestedSelector`

### Are these changes tested?

Yes (covered by existing tests)

### Are there any user-facing changes?

No

* Closes: #36892

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