[ECS] [Proposal]: ECS Service ordering

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**Tell us about your request**
What do you want us to build?

Recently, the new feature to start containers in a specific order was implemented in the scope of this issue https://github.com/aws/containers-roadmap/issues/123

But, this feature is only useful when a single ECS task has multiple containers with different apps. For example, Nginx and backend microservice.
In most cases, each ECS task has only a container for a single app. Therefore, it is not possible to use container orders between ECS services.

I think the ability to define order between ECS services would be very useful in the microservice world.
For example, applications for centralized configuration management should start before microservices such as Spring Cloud Config Server.
Also, it would be useful when performing the matenance of the ECS cluster which may cause restarting of services. (Changing ec2 instance type, changing AMI, etc)

**Which service(s) is this request for?**
ECS, ALB

Start by reviewing the referenced container-ordering issue #123 and the ECS/ALB service behavior described here. Define how dependencies between separate ECS services would work during startup and maintenance, and document the resulting proposal and completion criteria for the roadmap issue.

Written by the indexing model from the issue text.