Add an SPI to controller to allow sending extra metadata with paged requests

It would be nice to provide an interface for users to inject extra information (total number of items, total number of pages, etc.) into the header info returned during a paged request. We don't want to do that by default because, as discussed, there is no way for controller to know if there is an end to the data to make those calculations but an app developer may know that info and want to send it along to the client.