Raise custom PanderaValidationWarning instead of UserWarning

[nathanjmcdougall]

Discussed in #1271

^{Originally posted by marks-res July 20, 2023}
Just curious. If I understand correctly, the warnings system can be filtered by the type of the warnings class. Other code might co-opt UserWarning though. A Pandera specific warning could be more granular.

The plan would be to enable the current behaviour via a global configuration setting.