WebServiceWorkItemHandler should be extendable

The WebServiceWorkItemHandler is quite difficult to extend. Especially the CXF DynamicClientFactory (DCF) private attribute of the WIH is unextendable. I.e., we can't easily subclass WebServiceWorkItemHandler and configure a different DCF.

Proposal is to add a protected method 'buildDynamicClientFactory()', which returns the 'JaxWsDynamicClientFactory.newInstance()' in the default WebServiceWorkItemHandler implementation (basically a 'Factory Method' pattern implementation). This method should be used in the WebServiceWorkItemHandlers to retrieve the DCF.

This would allow us to override the factory method to provide different implementations in a subclass, for example an implementation which creates a DCF using a SpringBusFactory.