Compute's aggregate lists do not support resource's field selection

Aggregate lists' `fields` option only allows to select the following fields:

```
id
items
kind
nextPageToken
selfLink
```

Shouldn't selecting fields of the listed resources be supported as well?
It would be nice to be able to write a `field` option like: `items(selfLink),nextPageToken` that only returns the next page token and the selfLink field for each resource.
