Allow flexible result mapping for aggregation pipelines

[alcaeus]

Feature Request

Q	A
New Feature	yes
RFC	no
BC Break	no

Summary

When using aggregation pipelines in ODM, users can use QueryResultDocument objects to hydrate aggregation results into objects. However, when using more complex pipelines, this can quickly lead to issues. For example, https://github.com/api-platform/core/pull/2144#issuecomment-414065397 shows using a facet pipeline stage to simultaneously fetch results and the count from a collection at the same time. However, this can only be done with arrays and a separate iterator because dynamically mapping the aggregation result to an object is not possible.

The idea is to create a system similar to ORM's ResultSetMapping that allows for more flexible and dynamic mapping of aggregation pipeline results.