chore(python): Add py-polars to Cargo workspace

[stinodego]

This adds the py-polars crate to the workspace as a non-default member. Benefits:

• Integrates everything in the workspace. Can run everything from the workspace root (clippy / build / ...). This will help simplify the CI in the future (for now, everything keeps working as before).
• Removes duplication between dependency versions in workspace / py-polars manifest
• Only a single target folder in the workspace - saves a whole bunch of space on my device and saves re-compiling things.

The Cargo.lock is now in the repo root, as well as the profile definitions. These will still be picked up by maturin and included in the sdist.

For some context, the official recommendation for Rust projects is now to check in the lockfile, even for libraries:
rust-lang/cargo#12382