Aggregate functions in GraphQLList object

Addition of aggregate functions in GraphQLList Object such as in SQL:

Example:
In order to implement something like:
```sql
SELECT post_id, likes, count(likes) FROM posts;
```
> where likes stores the user_ids of users who liked a certain post

as
```GraphQL
query AllPosts {
posts {
post_id
likes
count(likes)
}
}
```

and for `GraphQLList` type data, provide a `sum(values)` function and so on...