Reference other services' resources from server side completions

This is a feature request.
# Usecase

`elb attach-load-balancer-to-subnets` API accepts
- load-balancer-name(`elb describe-loadbalancers`)
- subnet(`ec2 describe-subnets`)

and you want to autocomplete both parameters.
# Problem

At `completions-1.json`, `operation` assumes you call API of the same service, and subnet retrieval API belongs to a different service(ec2), you cannot define subnet resource outside of ec2.

```
"resources": {
"Subnet": {
"operation": "EC2:DescribeSubnets", <-- DOES NOT WORK
"resourceIdentifier": {
"Id": "Subnets[].SubnetId"
}
}
```