Support aggregation queries on computed fields for order_by

### Is your proposal related to a problem?

In v2.27, Aggregation queries for computed fields where added ([blog post](https://hasura.io/blog/introducing-aggregation-queries-for-computed-fields/)). This is an awesome improvement! However, while migrating an actual column to a computed field, I noticed that one feature was still missing: the ability to sort results using aggregation queries on computed fields.

Let's consider a similar query to the one on the blog post where we would list the authors with their average word count per article:
```gql
query {
users {
name
articles_aggregate {
aggregate {
avg {
word_count
}
}
}
}
}

```

if `word_count` **was not** a computed field, we could sort the users list by their average word count per article:
```gql
query {
users(order_by: {
articles_aggregate: {
avg: {
word_count: asc
}
}
}) {
name
articles_aggregate {
aggregate {
avg {
word_count
}
}
}
}
}
```

This is not possible as soon as `word_count` is a computed field.

### Describe the solution you'd like

Allow using aggregate on computed fields for `order_by` conditions.

Start by tracing the GraphQL aggregation and order_by paths represented by the query examples, focusing on how computed fields are handled in each. Done means users can sort with aggregate order_by conditions when the aggregated field is computed, while the shown aggregation query continues to work.

Written by the indexing model from the issue text.