[ECS] [request]: UpdateService call should return DeploymentArn of the created deployment

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**Tell us about your request**
The UpdateService API call currently returns "The full description of your service following the update call" in the form of a "service" object.
Since the new Blue/green paradigm has been introduced, I have been working to incorporate it into my CI/CD pipelines, which currently is somewhat brittle due to inability to conclusively match the deployment to the command that started it.

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

**Tell us about the problem you're trying to solve.**
In the pipeline, I have to get and store the ServiceDeploymentArn, to operate the hooks and other deployment scripts that get called during the deployment.

Calling update-service from AWS CLI doesn't produce any useful output that can be used in later in my pipeline script.
I have to call list-service-deployments and filter it by statuses PENDING and IN_PROGRESS, which can easily break the pipeline if, for example, someone else starts another deployment (e.g. from console) at the same time. That would produce 2 elements from the list call and I have no way of identifying which is which at this point.

The solution to that problem would be to add the ServiceDeploymentArn to the output of UpdateService call, so that I can store it first thing, and use it for subsequent operations on that pipeline.

No repository file or test is named. Start by reviewing the ECS UpdateService API response and the AWS CLI output described in the issue; the work is complete when the created deployment's ServiceDeploymentArn or DeploymentArn is returned and can be used by subsequent pipeline operations.

Written by the indexing model from the issue text.