List a Chewy category or search's refinement options

Returns the full set of refinement options (facets) a Chewy category or keyword search can be narrowed by -- brand, sub-category, lifestage, food form, breed size, special diet, health feature, flavor, ingredient, packaging type, price band, customer rating, and whichever other dimensions that listing exposes -- with each option's product count. No products are returned, so this is a much smaller call than chewy_category/chewy_search when only the option set is wanted. The Brand dimension is effectively Chewy's brand directory for the requested listing: each brand option carries its product count and, where Chewy publishes one, its own chewy.com/brands/<slug>-<id> landing-page slug in seo_hvf_slug. Exactly one of group_id or q is required. Every facets[].value paired with one of that facet's options[].value is a ready-to-use filter key:value pair for this endpoint, chewy_category, and chewy_search alike; passing filter here narrows the listing first, so the returned counts describe the already-narrowed result set. A group_id Chewy does not recognize returns a 404 rather than an empty option set.