sql: support pg_matviews virtual table

[awoods187]

An error pops up in Navicat when clicking materialized views:

v.ispopulated AS ispopulated, v.matviewowner AS mviewowner, v.definition AS definition, obj_description(c.oid) AS comment, t.spcname AS tablespace, v.schemaname AS schemaname 
FROM pg_matviews AS v 
INNER JOIN pg_class AS c ON v.matviewname = c.relname 
LEFT JOIN pg_tablespace AS t ON v.tablespace = t.spcname 
WHERE v.schemaname = 'public' 
ORDER BY v.matviewname ASC```

I180409 21:51:12.531915 380 sql/conn_executor.go:1805  [n1,client=[::1]:56172,user=root] execution error: relation "pg_matviews" does not exist

We don’t support materialized views at this time.