Always wrap statement in a named struct

In [GreptimeDB](https://github.com/GreptimeTeam/greptimedb/blob/v0.7.1/src/sql/src/statements/delete.rs), we write code like:

```rust
#[derive(Debug, Clone, PartialEq, Eq, Visit, VisitMut)]
pub struct Delete {
// Can only be sqlparser::ast::Statement::Delete variant
pub inner: Statement,
}
```

It's because in sqlparser-rs, these statement variants are unnamed struct:

```rust
Delete {
/// Multi tables delete are supported in mysql
tables: Vec,
/// FROM
from: Vec,
/// USING (Snowflake, Postgres, MySQL)
using: Option>,
/// WHERE
selection: Option,
/// RETURNING
returning: Option>,
},
```

I wonder if we can always wrap statement in a named struct so that downstream software can reuse the AST and impls (like for Display) more smoothly.

I don't know if it's a breaking change or we're generally OK with this.

Ref - https://github.com/GreptimeTeam/greptimedb/issues/3646

先檢視 sqlparser AST 中的 statement 定義，以及 GreptimeDB 的 src/sql/src/statements/delete.rs 中所示的下游 wrapper。檢查下游程式碼如何實作或使用 Display 與其他 AST trait，接著判斷包裝每個 statement variant 對相容性的影響。當有一份已記錄的設計或已接受的實作計畫，處理重用與 breaking change 方面的考量時，即視為完成。

由索引模型根據 Issue 內容生成。