Support sorting on `/grower-accounts`

By default, grower accounts are returned to the Admin Panel sorted by `id` in ascending order.
The current convention in the production Admin Panel is to list Growers in descending order by `reference_id` (a proxy for `created_at`) so that the newest growers show up first, as they're often the ones whose accounts require attention.

We need the query API to support this behaviour for continuity.