Narwhalify ArbitraryImputer

[davidhopkinson26]

What

make ArbitraryImputer polars compatible while maintaining backwards compatibility with pandas.

Why?

Making the project polars compatible for speed benefits. Narwhals allows us to do this piece by piece and maintain compatibility between components as it makes it possible to pass pandas dataframes to polars logic. This involves converting between pandas/polars dataframes.

How?

convert logic in ArbitraryImputer to function with polars dataframes and add narwhalify decorators to methods to ensure compatibility with pandas dataframes.

Update testing where required. This will require additional tests or parameterisation for polars workflows.