PERF: Series.str.split(expand=True) for pyarrow-backed strings
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/v2.1.0.rst` file if fixing a bug or adding a new feature.

Perf improvement for `Series.str.split` with `expand=True` for `ArrowDtype(pa.string())`:

```
import pandas as pd
import pyarrow as pa

N = 10_000
data = ["foo|bar|baz"] * N
ser = pd.Series(data, dtype=pd.ArrowDtype(pa.string()))

%timeit ser.str.split("|", expand=True)

# 93.8 ms ± 2.85 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)  -> main
# 5.43 ms ± 201 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)  -> PR
```
Nice! Thanks @lukemanley 