[ECS] [request]: Support healthCheckGracePeriodSeconds for EXTERNAL deployment controller

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**Tell us about your request**
Support healthCheckGracePeriodSeconds attribute for ECS services when deploymentController is EXTERNAL.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently you can define healthCheckGracePeriodSeconds to CODE_DEPLOY controlled ECS services but not for EXTERNALly controlled service. You need to define loadBalanced attributes but api does not support loadBalanced for EXTERNALly controlled ECS services.

**Are you currently working around this issue?**
Initially I did not give healthCheckGracePeriodSeconds. After I created my first PRIMARY task I updated the service. But I am using terraform as IAC. When I run terraform it removes the healthCheckGracePeriodSeconds attribute since I can not define it in configuration.

**Additional context**
Anything else we should know?

**Attachments**
Command:
```
aws ecs create-service --service-name test-service \
--cluster test-cluster --health-check-grace-period-seconds 900 \
--desired-count 1 --deployment-controller type=EXTERNAL \
--load-balancers targetGroupArn=,containerName=test-family,containerPort=80
```
Error:
```
An error occurred (InvalidParameterException) when calling the CreateService operation: LoadBalancers must be empty or null.
```

The issue names no repository files, tests, or implementation entry points. Start by reviewing the ECS CreateService constraints for EXTERNAL deployment controllers and how Terraform models the service; done means EXTERNAL services can retain and configure healthCheckGracePeriodSeconds without an unsupported loadBalancers setting.

Written by the indexing model from the issue text.