Select aggregate function result over relationship

[hrach]

Currently we are able to use aggregation function to select / sort results. There is also one aggregation function allowing to select -> count(stored)(). However, user is not able to select aggregation over relationships. We should add a method which will allow to fetch aggregation result from collection.

$user->books->toCollection()->getAggregation([ArgAggregateFunction::class, [YearFunction::class, 'publishedAt']]);

(YearFunction does not exists)