Related to cubicweb/cubicweb#831
Adds an endpoint available at
API_BASE_URL/possible-relation-targets to fetch all the possible entities as target of a relation.
Works for both subject and object relations. The user sends the source entity type, the wanted targets, and the relation type. This relation type may include the
reverse_ keyword to consider the source as object of the relation instead of subject.
If an eid is supplied, the endpoint will consider it for possible constraints.
Users can also send known values with the
known_values param to still compute constraints when creating a new entity (eg: you create a museum from a city, so you already know in which city that museum is even if it is not created yet).
- Unit tests
- Pagination (may leave this for another MR)