WFLY-1888 also load ServletExtension-s from parent class loader so that ...

...other modules can export services and add their own Handler-s

Say we want to add RequestCountHttpHandler via RequestCountHandlerExtension to support mod_cluster metrics on Undertow. Then an optional dependency can be added, e.g. ```<module name="org.wildfly.mod_cluster.undertow" services="export" optional="true"/>```

WDYT? Is this a reasonable approach, or should mod_cluster be handled completely differently? Actually, this might not be enough since I need to add channel conduit to track sent/received bytes too which I am not sure yet if can be done via ServletExtension.