ENH: Support string arguments for partition_cols in pandas.to_parquet
- [x] closes #27117
- [x] tests added
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

lgtm. @jorisvandenbossche or @TomAugspurger if any comments. @HawkinsBA Thanks!