[ DependencyInjection] Container::getServiceIds() missing from any interface

The public method Container::getServiceIds() is not declared in any interface. This method is used by several Symfony bundles both in test as well as in production code. It is also required in Drupal.

I guess an appropriate place for the method declaration would be in IntrospectableContainerInterface.

Could this go into 2.8 or should I only open up a PR for master?