Normally, when creating and updating (POST/PUT) an entity via the REST endpoints, you can use the URL of the relation target. (e.g.: send `{"package": "/packages/1"}` when `package` is a JPA `@OneToOne` relation).
Now this also takes into account when the JPA relation is annotated with `@JsonProperty` to change the serialized name.
Add unit tests for linkable associations.
Issue: #2165