BUG: avoid StringArray.__setitem__ to mutate the value being set as side-effect
Encountered this while working on https://github.com/pandas-dev/pandas/pull/51082, and this can be done as independent fix.

```
In [5]: arr = pd.array(["a", "b", "c"], dtype="string")

In [6]: value = np.array(["A", None])

In [7]: arr[[0, 1]] = value

In [8]: value
Out[8]: array(['A', <NA>], dtype=object)  # <-- value got mutated !!
```

I think in general the value that is being set should _never_ be mutated during the setitem operation.

- [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/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @jorisvandenbossche 