Add TypedPublishedEvents.matchingMapped(…)

[odrotbohm]

As the constraints to select events for verification are usually focused on the event's payload it would be nice if there was API to easily map to the content and register a Predicate on that to filter the owning event.

class SomeEvent {
  Payload payload;
}

// Before
events.ofType(SomeEvent.class).matching(it -> it.getPayload().equals(reference));

// After
events.ofType(SomeEvent.class).matchingMapped(SomeEvent::getPayload, reference::equals);