Add support for json_agg formatting

Please add support for json object formatting such that

```
json_agg(json_build_object('a', a.id, 'b', b.id, 'c', c.id))
```

is formatted as

```
json_agg(
  json_build_object(
    'a', a.id, 
    'b', b.id, 
    'c', c.id
  )
)
```

