Use Narwhals to support Polars and Modin DataFrames

[mwouts]

This PR add tests for Modin dataframes. It is on top of @DeaMariaLeon's PR to add support for more DataFrame types using Narwhals at #339 .

From this quick experiment I would say that the nbytes function (estimate the size of the dataframe in bytes) and the downsample functions (keep left and right most columns, top and bottom rows) could benefit a bit more from Narwhals.