Support PostConstruct callbacks in Extension class

It is sometimes useful to get to know after a portable extension class has been instantiated, so that the state of the Extension could be initialized [ie not establishing state in the constructor and having it called twice when the Extension is proxied]. Though the 1.0 specification does not require this, it would be useful to support @PostConstruct callback on Extension class in the next version of the specification.

Please see https://jira.jboss.org/browse/WELD-713 for additional information on this issue.