Adds stub of NarwhalsAdapter

[skrawcz]

The purpose of this adapter is to showcase how you can write
transforms that are agnostic of the dataframe type.

Notes

Assumptions narwhals has (I believe):

1. you can only have one "backend"; you can't mix & match. That means you can't load some in pandas, and some in polars I don't think.
2. This change uses the narwhals decorator. This assumes that non pandas/polars stuff would be left alone by it. If not, we could just skip adding it if we don't detect a type.
3. I think this is a good enough integration to get out -- we'll likely tweak/add more functionality as feedback comes in.

Changes

• adds narwhals plugin

How I tested this

• locally
• via integration test

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.