Returns the retailer slugs, display names, and known domains your
organization is currently entitled to use. This list is scoped to
the calling partner and can change over time (a retailer can be
disabled globally, or your organization's grants can change), so
treat it as the live source of truth rather than caching it
indefinitely.

domains lets you map a pasted product URL's host to a retailer
slug before calling identifier lookup, instead of hard-coding that
mapping yourself.