chore: flatten subset in unique one level up

[MarcoGorelli]

In general, it's probably good if we simplify the implementation of CompliantFrames by doing these kinds of operations at the Narwhals level

If we want to encourage people to extend Narwhals, it'll be easier if they just have to worry about subset: list[str] | None rather than subset: str | list[str] | None

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Related issue #<issue number>
• Closes #<issue number>

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

If you have comments or can explain your changes, please do so below

[FBruzzesi]

I think that's ok and I agree that for someone extending the API this is easier.

The only good use case to keep signature the same is for other methods to (re)use existing Compliant classes methods, yet it's fairly easy to spot what to pass with the fully annotated internals.

Ship it!

P.S. consider adding this gif?! 👀