BUG: Improve col_space in to_html to allow css length strings (#25941)
- [x] closes #25941
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Adds the ability to pass a string as the `col_space` parameter in `to_html`.

For backwards compatibility and similarity for other `to_*` functions, I kept in the ability to pass an `int`.  The `int` is automatically turned into `px` units (I chose `px` because that is the default if you put a unit-less CSS length into chrome).  If you want that removed and to always expect a string, let me know.