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>

src/PostgREST/Config/Database.hs

@@ -56,6 +56,7 @@ dbSettingsNames =
   ,"db_prepared_statements"
   ,"db_root_spec"
   ,"db_schemas"
+  ,"db_timezone_enabled"
   ,"db_tx_end"
   ,"db_hoisted_tx_settings"
   ,"jwt_aud"