Improve interoperability with Java

[whyoleg]

• Allow to provide ClassLoader when loading services
• Allow to get registered services via java.util.ServiceLoader directly without sweetspi.ServiceLoader
  • Because we allow to register objects, properties and functions as service providers, it might not be that easy
  • For @Service interface it's possible to just use delegation, for @Service abstract class it's not
  • decide if it should be possible to declare @ServiceProvider class
• Allow to get services via sweetspi.ServiceLoader which were not registered via sweetspi (?)