Add support for narwhals eager backends

First step in supporting Arrow tables in general, and any container that uses arrow tables in the background.

edit: renamed the issue to narrow the scope to narwhals eager backends like discussed on discord.

Start by locating the existing backend integration points and any current narwhals or Arrow-related handling in the repository. Define the eager backends that must be supported, then verify that tables and containers backed by Arrow can be processed successfully with coverage for each supported backend.

Written by the indexing model from the issue text.