Composer: Suggest the ServiceManager for the filters and validators

Since the ServiceManager component is only required for the `FilterPluginManager`/`ValidatorPluginManager` (and thus `FilterChain`/`ValidatorChain` and `StaticFilter`/`StaticValidator` as well, as they use the managers) it may be a good idea to suggest the ServiceManager component rather than require it (Zend\Validator) or not require it at all (Zend\Filter).

This would allow to use the filters and validators as standalone (e.g. by directly instantiating the classes) without having to download the ServiceManager component.