DelegatingWsConfiguration should load WsConfigurer lazily

[snicoll]

While reviewing #1391, I noticed that WsConfigurer beans are injected in DelegatingWsConfiguration, which forces them to be resolved before the low-level WS configuration takes place.

It would be better to inject an ObjectProvider and resolve them lazily. That would harmonize what we do elsewhere, e.g. in the cache abstraction of the Spring Framework.