Idea: support narwhals for easy support of multiple df libraries

[nathanjmcdougall]

https://github.com/narwhals-dev/narwhals

It might be possible to effectively provide support for a whole variety of other DF libraries without special case handling. In fact, it may be that supporting polars in a strategic way (by only using a subset of the polars DataFrame API) already provides the ability to do this, but it should be tested.