feat: add pre-config function

Allows using the in-database configuration without SUPERUSER

test/io/configs/expected/no-defaults-with-db.config

@@ -13,6 +13,7 @@ db-prepared-statements = false
 db-root-spec = "root"
 db-schemas = "test,tenant1,tenant2"
 db-config = true
+db-pre-config = "postgrest.preconf"
 db-tx-end = "commit-allow-override"
 db-uri = "postgresql://"
 db-use-legacy-gucs = false