Allow class realm manager delegates to alter public part of Maven core realm

As part of the fix for MNG-4747, a new class realm was introduced that represents the public part of the Maven core. Life for integrators that need to contribute additional classes like Polyglot Maven or M2Eclipse would be easier if this realm is also subject to the class realm delegates. This would allow to inject the custom types at a central place rather than injecting them into each and every plugin realm.