Add wrapper classes for Expression types

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

As part of the discussion in https://github.com/apache/datafusion-python/pull/1674 we found a user wants access to some of the elements within an expression. In this case, `Limit`. If there is a growing base of users who want to use `datafusion-python` to do things such as analyzing the plans and using the data in them, then we should probably add wrapper classes for each of the Expressions.

**Describe the solution you'd like**

Right now we have the expressions like `Limit` available in `datafusion.expr` reexported like

```python
Limit = expr_internal.Limit
```

Instead we could have a `datafusion.expr` module that contains

```python
class Limit(Expr):
```

which has attributes to get `fetch` and `skip`.

**Describe alternatives you've considered**

We could leave as is and continue to expose data members as requested.

**Additional context**

This is a follow on to https://github.com/apache/datafusion-python/issues/1673

先檢視目前的 datafusion.expr 匯出，尤其是 expr_internal.Limit 的重新匯出，然後閱讀 PR #1674 和 issue #1673，以了解其背後的使用情境。根據現有的 Expression 型別定義 wrapper 的範圍；當預期的 wrapper 公開所要求的運算式資料，包括 Limit.fetch 和 Limit.skip 時，工作即告完成。

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