Extract additional interface/API to tackle custom fields

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.

AC

• 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.