Expose Log4jContextFactory's ShutdownCallbackRegistry

Currently the instance created by org.apache.logging.log4j.core.impl.Log4jContextFactory#createShutdownCallbackRegistry in the default constructor for Log4jContextFactory is not acquirable without resorting to reflection. As such, any class which implements ShutdownCallbackRegistry is not able to get the actual instance of itself and must resort to either reflection or static methods to properly shutdown in an arbitrary lifecycle workflow.

This ask is to better clarify how a custom implementation of ShutdownCallbackRegistry can get its instance.