[ResourceBundle][Proposal] Target entity resolver

Now, The target entity resolver can find the interfaces in the container like model. It check if the configured classes or interfaces exists (it prevent error like #1113). The specs has 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 not a valid interface.