Migrate ManagedAgent state change to task

[fierlion]

Summary

This migrates the ManagedAgent state change events from the Container state change events to the Task-level state change events to support bugfix model changes.

Implementation details

This adds batched ManagedAgent events to the Task event

Testing

I tested manually by building agent/init on an exec-capable container-instance and running 2 separate tasks, one with a single container and another with 3 containers

aws ecs start-task \
    --endpoint-url <gamma endpoint> 
    --cluster gammaDefault \
    --task-definition exec-task:2 \
    --container-instances <instance-id> \
    --enable-execute-command

followed by:

aws ecs describe-tasks \
    --cluster gammaDefault \
    --endpoint-url <gamma endpoint> \
    --tasks <task-id> \
    --debug

I got the following output:

                    "managedAgents": [
                        {
                            "lastStartedAt": 1607631456.641,
                            "lastStatus": "RUNNING",
                            "name": "ExecuteCommandAgent",
                            "reason": "Execute Command Agent started"
                        }
                    ],

New tests cover the changes: Yes.

Description for the changelog

Migrate ManagedAgent state change from container to task event.

Licensing

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