ENH: Add strings_as_fixed_length parameter for df.to_records() (#18146)
This option changes DataFrame.to_records() dtype for string arrays
to 'Sx', where x is the length of the longest string, instead of 'O"

- [x] closes #18146
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
