Add concept of image to What's New API

In preparation for displaying images in the What's New UI for news items, the What's New API has been updated to support getting images for news items.

@Inject
@Named("xwikiblog")
private NewsSourceFactory factory;

@Inject
@Named("xwikiblog")
private NewsConfiguration configuration;

...
List<NewsSourceDescriptor> descriptors = this.configuration.getNewsSourceDescriptors();
NewsSourceDescriptor descriptor = descriptors.get(0);
List<NewsSourceItem> items = this.factory.create(descriptor.getParameters())
  .forUser(...)
  .forCategories(...)
  .build()
String imageURL = items.get(0).getImageURL().get();
...