PHPORM-106 Implement pagination for groupBy queries

[GromNaN]

Fix PHPORM-106

Pagination needs 2 queries:

1. Get the total count
2. Select the items for the specified page using offset/limit.

For 1, Eloquent wraps the raw SQL into an other SQL queries. For MongoDB, we use an aggregation pipeline for groupBy queries. Getting the total count is as simple as adding a $count stage to the pipeline.
For 2, nothing to do, the pagination leverages the query builder offset/limit methods that already works with aggregation pipeline.

I have to explicitly restrict usage of the pagination on distinct queries, as the Collection::distinct() method doesn't support counting.