feat(mongodb): Add pagination metadata to the aggregation results

[GromNaN]

Q	A
Branch?	4.1
Tickets	Closes #6878
License	MIT
Doc PR	-

Simplifies code by removing the need to extract firstResult and maxResults from the pipeline, by adding a $setFields stage to add the litteral informations.

This also remove references to the MongoDB Cursor and the UnitOfWork stored in the Paginator class. I hope this helps preventing memory leak issues.

Since I'm modifying the aggregation pipeline, the parameters of the Paginator constructor and deleting 2 constants from the Paginator class, it's best not to merge this as a bugfix.

[GromNaN]

This constants are removed. They should have been marked as internal. We can keep them for backward compatibility, but they are useless now that they are not used in the aggregation pipeline.

[soyuka]

It looks like the patch is responsible for the failing CI

[GromNaN]

Required for mocking AddFields. See release 2.9.2

[GromNaN]

Thanks to Behat tests. I had totally misunderstood the meaning of limit=0. I thought it meant “infinite”, whereas it actually means zero (I think it should be a error to put zero results per page, but maybe there's a use case).

So, to solve this problem, I add an empty array directly to the results rather than building a query that returns nothing.

[soyuka]

Thanks @GromNaN !