ENH: Series.str.join for ArrowDtype(pa.string())

[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.

Adds support for Series.str.join with ArrowDtype(pa.string()):

In [1]: from pandas import Series, ArrowDtype

In [2]: import pyarrow as pa

In [3]: ser = Series(["abc", "123", None], dtype=ArrowDtype(pa.string()))

In [4]: ser.str.join("-")
Out[4]: 
0    a-b-c
1    1-2-3
2     <NA>
dtype: string[pyarrow]

This is already supported by string[python] and string[pyarrow] as well as python strings in general:

In [1]: "-".join("abc")
Out[1]: 'a-b-c'

This is work towards being able to add ArrowDtype(pa.string()) to the string benchmarks.

[mroeschke]

Nice find thanks @lukemanley