refactor: pass ExprMetadata down to compliant Exprs from narwhals.Expr

[MarcoGorelli]

Currently, we track expression metadata at the compliant level:

• whether it aggregates, changes length, transforms
• the function name

This is quite error-prone, as we repeat logic in quite a few places.

I'd prefer it if:

• we didn't track the function name at all for most backends. It's only really needed for pandas/pyarrow/dask because of limitations in their group-by APIs, so we should only do it there
• one of the metadatas we could track is what kind of column selection we start with (nth, col, selector, all). Because in group_by-agg, selector and all require some special treatment. But this could be done much more simply at the Narwhals level, without tracking function names everywhere in DuckDB / PySpark / Ibis / whatever else we add, which I hope would also have a modern enough syntax

To do this, I haven't tried, but perhaps:

• _to_compliant_expr could take a metadata argument, which takes a TypeDict of metadata which we keep track of at the narwhals/expr.py and narwhals/functions.py level
• compliant expressions just read from the metadata passed by above to determine how and whether to broadcast