`DROP` statement should be grouped by

[LucaCappelletti94]

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