add: config db-timezone-enabled for optional querying of timezones

To avoid repeated querying of `pg_timezone_names` every time schema
cache is reset, `Prefer: timezone` can be disabled by setting
`db-timezone-enabled = false`.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

test/io/configs/no-defaults.config

@@ -18,6 +18,7 @@ db-root-spec = "openapi_v3"
 db-schemas = "multi,   tenant,setup"
 db-config = false
 db-pre-config = "postgrest.pre_config"
+db-timezone-enabled = false
 db-tx-end = "rollback-allow-override"
 db-uri = "tmp_db"
 jwt-aud = "https://postgrest.org"