[Request] Support IAM role import for restricted environments.

[bvtujo]

Some customers don't allow individual teams or contributors to create IAM roles in their AWS environments. We've had at least one customer say that the fact that Copilot creates lots of IAM roles is a blocker for their use case, as they need to create roles through a centralized system.

If possible, we might wish to support importing IAM roles for the app, env, and service level.