Improve query and aggregator classes

[alcaeus]

Q	A
Type	improvement
BC Break	no
Fixed issues	Fixes #2699 (cc @soyuka)

Summary

To improve handling of query and aggregation classes, a new joint IterableResult interface is introduced. This method includes the execute, getIterator, and getSingleResult methods that are common to both queries and aggregations. In addition, this allows us to keep the actual classes final, while allowing other platforms to mock what's returned from the builders for testing purposes.

Note that while the return type of getAggregation and getQuery in the builders has widened, I've added an @return statement to indicate that the methods themselves still return the same class instances that they returned previously. However, changing the return type is necessary to allow the mocking described above. So while this is technically a BC break, in practice it has no effect, especially as any class extending the builders would still be allowed to use the narrower return type.

[GromNaN]

I think the issue it that Aggregation\Builder::execute() should not be deprecated. This was the perfect method for a mock.
As you can see in GromNaN/api-platform-core#2, the alternative is more verbose, that's not good for DX.

With this solution, you have to rely on the phpdoc to give the exact return type, which is a regression. Can we revisite #2211 instead?

[GromNaN]

Noted that this is necessary for static analysis and autocompletion, but we don't use it as return type to allow mocking more easily.