Backend: when refreshing shipping rates, include backend-only shipping methods

Fixes #5755 
Rather than true false, I would prefer to see a constant being passed in.

Which is easier to understand on first glance?

```Ruby
shipment.refresh_rates true
shipment.refresh_rates false
```

```Ruby
shipment.refresh_rates ShippingMethod::FRONT_END
shipment.refresh_rates ShippingMethod::FRONT_AND_BACK_END
```