RFC: Refactor functions into Expressions namespaces and functions on the Expr directly

One thing I disliked a lot about pyspark was the Functions. Since most of if not all DataFusion.functions take in an expression and return an expression, we could rework this in the Expr namespaces, this should help finding the right expressions much easier, some examples:

- `col().dt.current_date()`
- `col().list.ndims()`

I am happy to start working on this and creating a full list of how the Expr and Expr.namespaces can look like