Provide progress feedback

When running a long data migration, it's not possible to know its progress. Shopify's maintenance_tasks has a mechanism to calculate that automatically (via count + time each tick takes).

We could start with a lower-level API just to let developers manually report the progress of a task. In the future, we could mirror maintenance_tasks's API of defining a collection and a method to process each item. That could use the low-level API under the hood.