[Credits] Return reference in List account entities endpoint

The Credit an account and Debit an account endpoint are allowing both to provide a reference to link this transaction to an event in the customers system. But the list account entries endpoint is not returning this reference making it impossible to associate the returned entries to e.g. an transaction event.

Expected: Return the reference in the list account entries endpoint.

Workaround: Store the transaction_id app side (Which does not solve the confusion around the existing reference field on the Credit an account and Debit an account endpoints)