[9.x] Expose next and previous cursor of cursor paginator

[gdebrauwer]

The json response of a CursorPaginator does currently not expose the raw values of the next cursor and previous cursor (it only exposes full URLs), while that is something that should be exposed by default in my opinion. This is similar to the LengthAwarePaginator, where the values of the current / next / last page are also exposed.

[taylorotwell]

Think it makes sense to serialize them as next_cursor and prev_cursor instead?

[gdebrauwer]

I chose to use _page suffix because of the existing next_page_url / prev_page_url fields, but you are probably right that it is better if we use the _cursor suffix for these new fields 👍