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

先閱讀現有的 DataFusion Python Functions 與 Expr APIs，接著盤點哪些函式接受並回傳 expressions。將該盤點結果與提議的 Expr 和 Expr namespace 範例進行比較。完成的條件是就完整的 API 設計及相應的實作範圍達成共識；此 RFC 不指定檔案或測試。

由索引模型根據 Issue 內容生成。