https://github.com/stac-utils/stac-geoparquet/blob/main/spec/stac-geoparquet-spec.md#including-a-stac-collection-json-in-a-stac-geoparquet-collection

I think we'd support this on the way in either by:

• A builder, or
• Expecting it on the input item collection (cleaner, I think)

Way out would be similar-ish, where if we didn't do it on an item collection we'd just hit the parquet file twice, which seems bad.

Probably just add a collection field to an ItemCollection, eh?