[ResourceBundle][Proposal] Target entity resolver

Now, The target entity resolver can find the interfaces in the container like model. It checks if the configured classes or interfaces exist (it prevent error like #1113). The specs have been improved too.

I have a problem with the spec `it_should_get_interfaces`  I can not use the `spec\Sylius\Bundle\ResourceBundle\Fixture\Entity\FooInterface` because `interface_exists()` does not consider that is a valid interface.

Useful when you use the resource bundle standalone.