ENH/TST/DOC: set infer_nrows for read_fwf (GH15138)
- [ ] closes #15138
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

`read_fwf` can infer the widths of columns of a dataset from a file or buffer, but if the widths change after the first 100 rows, then the default behavior fails to capture the data correctly. This PR addresses that issue by allowing the user to set a parameter `infer_nrows` that is passed down to the appropriate function.

The associated test ensures that `colspecs` can be determined from the first row, thereby missing data from the subsequent rows and verifying the expected behavior.