Add ecs interface to ecs shared library in ecs-agent.

[RichaGangwar]

Summary

Add the ECS client API interfaces to shared library under ecs-agent/api/ecs folder.

Implementation details

1. Added a new file interface.go with relevant interfaces to ecs-agent/api/ecs.
2. Moved the ecs api model files from ecs-agent/ecs to ecs-agent/api/ecs.

Testing

make test

New tests cover the changes: No

Description for the changelog

Add ecs interface to ecs shared library in ecs-agent.

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

No

Licensing

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

[timj-hh]

Do we need to update the /ecs-agent/vendor as well here?

[RichaGangwar]

Do we need to update the /ecs-agent/vendor as well here?
I had run go mod tidy and go mod vendor on both agents to update, but looks like there is nothing to be updated to the vendor directory.