Add exists method to PaginatorTrait with tests

[IgnisDa]

Introduce an exists method to the PaginatorTrait to efficiently check for record existence.

Fixes #730.

[Expurple]

Thank you! I'll review this a bit later.

TODO for myself: also merge this in master (2.0.0).

If you're ok with using your own unreleased fork of 1.1.x until 2.0.0 comes out, you can change the base of the PR to master and we'll merge this only in master.

[IgnisDa]

Thank you! I'll review this a bit later.

TODO for myself: also merge this in master (2.0.0).

If you're ok with using your own unreleased fork of 1.1.x until 2.0.0 comes out, you can change the base of the PR to master and we'll merge this only in master.

Can this be released as a patch 1.1.x release? I think it should be fine since it does not have any breaking changes. That way I can continue to use the crates.io version.

[Expurple]

Yeah, I assumed that you may want that. I'm not the one making (the decisions about) releases, but @tyt2y3 will probably do this

[IgnisDa]

@tyt2y3 Can you take a look at this please? Thanks!