ENH: Implement StringArray.min / max
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Using the new masked reductions from @jorisvandenbossche to implement these for StringArray. Part of https://github.com/pandas-dev/pandas/issues/31746 but doesn't close because we're not adding sum here.