 assignee and assigner should be an `@id` on serialization

# Bug Report

## Describe the Bug

In `JsonObjectFromPolicyTransformer` the assignee and the assigner as serialized as `@value` while in the ODRL spec are expected to be an `@id`

if no versioning scheme is applied or other form of backward compatibility this will be a breaking change when fetching the `ContractAgreement` in the `policy` field, as it will be serialized as `@id`
