Implementing a new check

Each database structure check starts with an SQL query to the pg_catalog.

SQLFluff is used as a linter for all sql queries
All queries should be schema-aware, i.e. we filter out database objects on schema basis:
```
where
    nsp.nspname = :schema_name_param::text
```

All tables and indexes names in the query results should be schema-qualified. We use ::regclass on oid for that.

select
    psui.relid::regclass::text as table_name,
    psui.indexrelid::regclass::text as index_name,

Provide feedback