ENH: loosen XLS signature
- [x] closes #41225
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Added ability to check for multiple XLS signatures according to testing files available at [Spreadsheet Project](https://www.openoffice.org/sc/testdocs/). Also defer raising an error when the engine is specified but the file signature does not match one of the values in `SIGNATURES`: this allows a user to attempt to specify and use an engine even if the passed file doesn't match one of the provided values in `SIGNATURES`.

This is my first PR for this project, so please let me know if more is expected (writing tests, writing a whatsnew entry, etc). I have run Flake8 on the code and successfully opened BIFF2 through BIFF8 files with this method. Thanks!
