Add support for `ALTER INDEX` queries.

[igalklebanov]

Hey 👋

As pointed out by Alzoom on discord, alter index is a query that's supported in postgres, mssql and oracle.

Dropping and recreating an index, if possible, should probably be avoided since it can take a long while with huge datasets - so this comes in handy. The actions that can be taken are quite advanced and rare (so low priority) - except for rename index. We could support rename, and leave an expression method for the rest (not sure how much value such a thing has against raw sql here tho).