Currently, we have hardcoded field definitions in the PageAggregator but we should provide our users with an easy way to write some logic to achieve aggregation with their custom fields.
- Have an API to extract one field per class e.g. CompositeFieldAggregator
- Each aggregator knows about the type(s) they can handle
- Aggregators are multibinded and traversed over until one aggregator feels responsible to respond.