`DROP` statement should be grouped by

Hi,

I noticed that while there exist a more generic `Drop` variant, there also exist separate `DropFunction`, `DropDomain`, `DropSecret` etc.. and I believe that these variants could and should be folded into the more generic `Drop` variant.

Do let me know whether you also believe this to be the case.

I may take care of this in a future PR when I get the time.

Best,
Luca

首先定位 Rust AST 中通用 Drop 变体以及独立的 DropFunction、DropDomain 和 DropSecret 变体的定义。检查这些变体的解析和表示方式，然后将它们合并，并验证现有的 SQL 解析行为仍然正确。

由索引模型根据 Issue 内容生成。