[11.x] Support DB aggregate by group

[GromNaN]

Currently, the aggregate methods ignore the group by clause. This PR change the result of

$builder->from('users')->groupBy('role')->count()

to return a Collection of [role, count] instead of a single total count.

- select         count(*) as aggregate from "users"
+ select "role", count(*) as aggregate from "users" group by "role" 

Result before: 40
Result after:

role	aggregate
admin	10
user	30

The aggregate result is always in a column named aggregate and the "group by" columns are added to the result.

Why?

I'm implementing withCount for MongoDB models and hybrid MongoDB to SQL models. This implementation is done during eagerLoad by doing a count by foreign key in the related table (i.e. select foreign_id, count(*) from mytable where foreign_id in (1,2,3) group by foreign_id). I managed to do that for MongoDB, and this PR makes it work for hybrid MongoDB to SQL models.

Union and group by

This also works with unions. I have to change the generated SQL to apply the group by clause to the union result instead of the first query of the union.

For the following query:

$builder->from('users')
    ->union($this->getBuilder()->from('members'))
    ->groupBy('role')
    ->aggregate('count')

The generated SQL is before/after the PR:

- select count(*) as aggregate from ((select * from "users" group by "role") union (select * from "members")) as "temp_table" 
+ select "role", count(*) as aggregate from ((select * from "users") union (select * from "members")) as "temp_table" group by "role"

If anyone need to generate the previous SQL, the query builder for the 1st subquery would have to be wrapped:

  $builder
-    ->from('users')
-    ->groupBy('role')
+    ->from($this->getBuilder()->from('users')->groupBy('role'), 'alias')
     ->union($this->getBuilder()->from('members'))
     ->aggregate('count');

Return type of count/sum/avg/min/max

The return type of the aggregate methods indicate that a single value is expected: the result of the aggregation. But since the group by clause is used, the result is a collection of [...groups, aggregate]. I had to change the return type in phpdoc to add Collection. This may impact the count function only, as other return mixed that already includes Collection.

An alternative could have been to add a new methods sumBy($function, $column, ...$groups) but I think this is already the role of ->groupBy(...$groups) to specify the expected groups.