Add a method to retrieve the identifier values in the Common interfaces

Currently, https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Doctrine/Security/User/EntityUserProvider.php is tied to the ORM, forcing bundle integrating other Doctrine projects to reimplement it.

The only place which is not (or cannot be) implemented using only methods from the Common interfaces is retrieving the current id. `getIdentifierValues` is now implemented by the ClassMetadata in all ORM, MongoDB and CouchDB but not in the interface.

Note that the tricky point about adding it is the fact that the DisconnectedMetadataFactory (used when generating entities) returns a ClassMetadataInfo instead, which does not contain this method (as it relies on the existence of the class) but implements the interface.