Add tests for every contract endpoint.

[ainmosni]

This adds tests for every contract HTTP endpoint on both the provider
and consumer side. These tests are based on the examples in the
IDSA dataspace specification.

The average test goes as follows:

1. Create a starting state.
2. Submit the message to the correct endpoint.
3. Inspect if the response is correct.
4. Inspect if the storage is in the desired state.
5. Inspect if the right request will be sent out to the reconciliation
   loop.

These tests also exposed some small bugs that are now fixed:

• The reconciler is now an interface to make testing possible.
• SetConsumerPID now actually sets the consumer PID.
• Unify the termination handler as they are on the same path
  for both the consumer and provider flows.
• Fix some ODRL validators.
• Fix the JSON-LD unmarshaller, as it put a nil where it shouldn't be.