Add BackendStatusString method to ContainerStatus

[amogh09]

Summary

Currently ContainerStatus enum and backend recognized container status strings are coupled as BackendStatus method that is used to map former to latter returns a ContainerStatus and not a string. This is not ideal as the coupling restricts the evolution of internal container statuses in Agent. This PR decouples the two by introducing a new BackendStatusString method for ContainerStatus type that maps the internal container status to a suitable backend recognized status string. Existing BackendStatus method is marked as deprecated in this PR.

BackendStatusString method is slightly different from BackendStatus in that its steady state parameter is optional and defaults to ContainerRunning. That will be useful in cases where there is no special steady state for a container.

This decoupling already exists for TaskStatus type, this PR extends the same logic to ContainerStatus.

amazon-ecs-agent/ecs-agent/api/task/status/taskstatus.go

Lines 69 to 78 in b702281

	// BackendStatus maps the internal task status in the agent to that in the backend
	func (ts *TaskStatus) BackendStatus() string {
	switch *ts {
	case TaskRunning:
	fallthrough
	case TaskStopped:
	return ts.String()
	}
	return "PENDING"
	}

The new method is not currently used but we plan to use it in the near future.

Testing

New tests cover the changes: yes

Description for the changelog

Enhancement: Decouple internal container status and backend recognized container status by introducing BackendStatusString() method to map internal container status to a backend recognized container status.

Does this PR include breaking model changes? If so, Have you added transformation functions?

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[JoseVillalta]

late comment but this LGTM