Introduce a way to map a single entity type to multiple indexes:

• on bootstrap, user code will provide a list of index names that this entity type will be mapped to (instead of just one usually).
• at runtime, each entity to index will be inspected by user code upon indexing, and routed to the correct index.

For example, we could repurpose the RoutingKeyBridge, renaming it to a RoutingBridge. Its APIs would change a lot, but something like this could do:

public class MyRoutingBridge implements RoutingBridge {

  private final IndexReference indexReference1; // Obtained by the binder
  private final IndexReference indexReference2; // Obtained by the binder

  @Override
  public void route(Route route,
      String tenantIdentifier, Object entityId, String documentId, Object entity, 
      RoutingBridgeRouteContext context) {
    route.index( indexReference1 ); // Only necessary if there are indexes declared by the binder
    route.routingKey( <compute some routing key> ); // Only necessary if the binder enabled routing keys
  }
}

This should address:

• Part of the use cases mentioned in HSEARCH-3313: essentially all the use cases except those where the list of indexes is not known in advance. Even that one could be covered eventually, but let's keep it for later.
• Most, if not all, of the use cases for Indexing Event Interceptors ( HSEARCH-3108 Open ). These interceptors are mainly used to prevent indexing of entities that are in a certain state. We could provide a way for users to route an entity to "nowhere" (route.discard()?), meaning it will be removed from all indexes and will not be added anywhere.