Added diff method to AuditReaderInterface

This PR must wait for this PR on the SonataDoctrineORMAdminBundle:
https://github.com/sonata-project/SonataDoctrineORMAdminBundle/pull/365

This change was (currently is) implemented in this PR but will be removed in the following PR because it is no longer needed there:
https://github.com/sonata-project/SonataAdminBundle/pull/2266