Add support for ALTER TABLE DROP INDEX

[vimko]

This PR adds support for mysql's drop index:

• DROP INDEX idx_a ON table_a
• ALTER TABLE table_a DROP INDEX idx_a

Addresses #1863

[iffyio]

Thanks @vimko! fyi there's a similar PR here for DROP INDEX idx_a on table_a feature, we'll probably look to land that one and rebase this on top of it

[vimko]

@iffyio Indeed, I didn't notice that. Thank you very much for reminding me.

[iffyio]

@vimko could you take a look at the cargo failure when you get the chance?

[vimko]

@vimko could you take a look at the cargo failure when you get the chance?

@iffyio Thank you for the reminder, I have already taken care of it.

[iffyio]

LGTM! Thanks again @vimko!
cc @alamb