Cursor could return the existence of next and previous pages

[garryod]

It would be really convenient if the Cursor was able to tell us about the existence of rows preceding or succeeding our current page. This would improve upon the current "convenient" method of making multiple SQL queries, and the less convenient method of building a query with sea-query.

The implementation typically consists of fetching an additional row either side of the page, and checking for their existence to deduce the existence of another page, as detailed in this blog post.

In my mind, the API may look something like:

let (items, has_next, has_previous) = Entity::find()
    .cursor_by(Column::Id)
    .after("SomeKey")
    .first(42)
    .all_with_neighbours(&db)
    .await?;

I'd be happy to make a PR to implement this if there is demand

[garryod]

I've been playing around with implementing this and am very close to a working solution but have come across one issue I can't seem to resolve. Is there a way to test equality between the ValueTuple used as the "cursor" ("SomeKey" in the above) and the returned SelectorTrait::Item? If this isn't possible are there any alternative approaches you might recommend?