Add support for Vertica

This is not an issue, but probably an opportunity to serve the Vertica customer base. The tool works beautifully with Vertica using SQL view. HP Vertica does not come with a GUI client and other ones out there (DBeaver, DBVisualizer) are not as cool, simple and fast as Postico. Ofcourse I am biased because I am not a heavy user of these other tools.

No Schema/Table view shows up when connecting to the Db
`Postico[39519]: Failed to load children of : Error Domain=at.eggerapps.Postico Code=1 "Query failed" UserInfo={PG_DIAG_SOURCE_FUNCTION=throwRelationDoesNotExist, PG_DIAG_SEVERITY=ERROR, PG_DIAG_MESSAGE_PRIMARY=Relation "pg_namespace" does not exist, NSLocalizedRecoverySuggestion=PostgreSQL said: Relation "pg_namespace" does not exist, PG_DIAG_SQLSTATE=42V01, query=SELECT oid, nspname, nspname = ANY (current_schemas(true)) AS is_on_search_path FROM pg_namespace, NSLocalizedDescription=Query failed, PG_DIAG_SOURCE_LINE=3328, PG_DIAG_SOURCE_FILE=/scratch_a/release/24526/vbuild/vertica/Catalog/CatalogLookup.cpp}`

Start by reproducing the schema/table-loading failure when connecting to Vertica and inspect the reported query against pg_namespace. Determine which database-specific catalog behavior prevents the schema view from appearing; done means Vertica connections expose their schemas and tables without this query failure.

Written by the indexing model from the issue text.