ast: provider builders for convenient programmatic construction

I have a use case where I'm generating SQL strings, and in general having `sqlparser::ast` is quite good, but the structs are particularly painful to construct programatically. It'd be interesting if the ast structs offered an optional feature that had https://github.com/elastio/bon builders derived to build them.

Maybe somebody will teach me how to safely construct strings that I can then parse (that'd be fine! I just am not generally eager to be doing string formatting to produce sql, particularly given I don't have full control over the identifiers). For now I am generally just doing this formatting, but I don't love it.

该 issue 将 sqlparser::ast 和一个由 bon 派生的可选 builder feature 列为入口；检查当前 AST 结构体和可选 feature 的定义方式。完成后应为相关 AST 结构体确定一种达成共识且可用的 builder 方案，从而减少手动格式化 SQL 字符串的需要。

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