Getting the value for a header GUC on PostgreSQL v14 is done using `current_setting('request.headers')::json->>'name-of-header'` and in a similar way for `request.cookies` and `request.jwt.claims`
PostgreSQL versions below 14 can opt in to the new JSON GUCs by setting the `db-use-legacy-gucs` config option to false (true by default)
Now that PgVersion is not part of DbStructure, Config is a more apt
module for it.
Also rename getDbStructure to queryDbStructure. AppState also had a
getDbStructure function for a record field.
