Support for running a subprocess with AWS role assumed

Unfortunately, some computer programs don't have good support for assuming roles (particularly with MFA). It would be helpful if the `aws` CLI had support for running a subprocess with a role pre-assumed and the correct env vars set. You could imagine usage like:

```
$ aws --profile=admin exec terraform apply
```

This feature request was inspired by this comment: https://github.com/terraform-providers/terraform-provider-aws/issues/5078#issuecomment-499855094

Which requires playing in the CLI's implementation details, which seems unfortunate. If this functionality was simply included with `aws`, it'd be a real time saver.