Support for "self" links in relationships #20
Description
JSON:API supports "self" links in relationships that can be used to fetch resource linkage (https://jsonapi.org/format/1.0/#fetching-relationships).
We should provide methods to transparently fetch these resource identifiers and treat them like the identifiers in data members of relationships. Alternatively we could provide hooks for the user (e.g. in Context) to translate these resource identifiers into actual Resource objects.