Add support for java-based configuration at SpringWebProcessingContextBuilder

[danielfernandez]

Currently, SpringWebProcessingContextBuilder only allows configuring beans by means of setting the location of an XML file.

But XML configuration is now on its way to deprecation so we should allow the configuration of Spring's ApplicationContext by directly specifying beans or, ideally, @Configuration classes.

[pmihalcin]

@danielfernandez
Is it still the case that this library won't recognize @Configuration classes?

I'd like to start using it with @WebMvcTest spring boot annotation, but I'm struggling to get it up and running.