Support External Launch Type

[RobertNorthard]

As an architect, I would like copilot to support the ECS Service / Task External launch type so that I can demonstrate creating a consistent deployment experience for a workload stretched across on-premises and AWS. At the moment, copilot can be used to create the ECS environment and deploy a Fargate based service but is not able to create a service with an External launch type, resulting in an inconsistent deployment mechanism.

Related issue for EC2 launch type support: https://github.com/aws/copilot-cli/issues/1094