Add support for custom where predicate and index type

[manwantnosmoke]

Motivation

Creating a partial index is impossible right now, but adding support for both index type and where predicate would help creating an index like this:

CREATE UNIQUE INDEX cake_filling_id_idx ON public.cakes USING btree (filling_id) WHERE (filling_id IS NOT NULL);

[billy1624]

Hey @manwantnosmoke, would be nice to have. A PR would be welcomed :)