Add ACS attach resource responder to ecs-agent

[danehlim]

Summary

Add ACS attach resource responder and other associated code it is dependent on to ecs-agent/. This ACS message responder handles attach resource messages (i.e., type ConfirmAttachmentMessage) from ACS. Resources associated with such messages are non-ENI and can be specific to Fargate launch type.

For clarity, this pull request just adds functionality to the ecs-agent module and does NOT change the agent module in any way to use this functionality. That is, in this pull request, the ACS attach resource responder defined in ecs-agent/ is NOT integrated into our Agent defined in agent/.

Implementation details

1. Add attach resource responder to ecs-agent/acs/session/ directory
2. Add resource models which attach resource responder is dependent on to ecs-agent/api/resource/ directory
3. Update ecs-agent/metrics/constants.go with constants specific to attach resource responder

Testing

Unit, integration, and functional tests.

New tests cover the changes: yes

Description for the changelog

Add ACS attach resource responder to ecs-agent

Licensing

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