Use identifiable structs for AST variant fields

When consuming the generated AST for further processing, one needs to match against and deconstruct `ast::Statement` instances. Currently, the fields of this enum's variants are defined inline... which makes it impossible for downstream code to represent the nested structures as types. For example, consider this structure:

https://github.com/ballista-compute/sqlparser-rs/blob/35ef0eee3857c59827b2587bc813218a00dce264/src/ast/mod.rs#L490-L497

I propose we migrate to the following pattern.
```rust
pub struct AnalyzeStatement {
table_name: ObjectName,
partitions: Option>,
for_columns: bool,
....
}
pub enum Statement {
Analyze(AnalyzeStatement),
...
}
```

Obviously this is a massive breaking change, but that seems OK given the `0.9.x` crates.io version.

Thoughts?

首先查看 src/ast/mod.rs 中的 Statement enum 及相关定义，尤其是 490-497 附近链接的行。评估如何将每个 inline variant field 转换为可识别的 struct，同时保留所提议的 AST 形状。完成标准是相关的 AST variant 使用命名 struct，并且由此产生的 breaking API change 得到一致应用。

由索引模型根据 Issue 内容生成。