Improve auto-completion candidates for Postgres-like DBs

Currently the query for auto-completion candidates is very rudimentary, i.e., it looks only for tables via the information_schema. That means it misses views, materialized views, fdw, and probably more.

For Postgres-like DBs there's much better and more complete ways to find completion candidates. Something along the lines of

select ... from pg_class where relkind in ('r','v','m') ...;

There's lots of room for improvements. I'll be happy to discuss these in case of interest.

Thanks for the request, @cstork! We've logged this internally and I'll update this post when there is any additional information.