Refactor dataframe support to use narwhals generic dataframes

[fubuloubu]

Overview

We force our users to use pandas dataframes to reduce a dependency on polars, which is less widely supported than polars. https://github.com/narwhals-dev/narwhals is a really cool project that makes a generic compatibility layer that would just let us delegate to whatever the user has installed in order to convert the dataframes into the format they desire. It is also particularly cool in that it supports lazy dataframe operations universally, which could be useful for ape users.

Specification

TBD

Dependencies

n/a

[linear]

APE-1729 Refactor dataframe support to use narwhals generic dataframes

[fubuloubu]

https://narwhals-dev.github.io/narwhals/api-reference/narwhals/