PERF: Series.str.split(expand=True) for pyarrow-backed strings

[lukemanley]

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• 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

[mroeschke]

Nice! Thanks @lukemanley