Returns metadata for every custom (uploaded) FHIR profile on this
instance, across all implementation guides. The full StructureDefinition
JSON is omitted from each entry; fetch a single profile by id to retrieve it.

The url query parameter filters by canonical URL. The canonical URL is the
stable key other platform features use to reference a profile (FHIR's
meta.profile, baseDefinition), since StructureDefinition ids are only
unique within a package. A non-matching filter returns an empty list, not a 404.