Provide configurable timeout for ExternalContextObjectFactory.createContext

An ExternalContextObjectFactory's createContext method might take a long time to complete, because usually it will communicate over the network. It would be useful to be able to cap the waiting time, and there should be a timeout enabled by default – it might happen that the remote server will get stuck during the context creation and the external context will never be successfully created (or it will just take too much time). If a timeout occurs, createContext can for example return null, or throw an exception, not sure which is better.