`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

Start by locating the Rust AST definitions for the generic Drop variant and the separate DropFunction, DropDomain, and DropSecret variants. Review how these variants are parsed and represented, then consolidate them and verify that existing SQL parsing behavior remains correct.

Written by the indexing model from the issue text.