Make it possible to influence the decision to index something

Currently, if you have "defer_indexing" set to true, the module will queue the entity even if the entity type would not end up getting indexed. This clutters the queue.

It can also be useful to be able to influence this decision if you have a site with millions of entity updates, so that you can decide to not queue an item if it is already in the queue.

I suggest adding a hook to alter the decision to index. This would take care of both cases mentioned above.