[ECS/CloudFormation] [request]: Assign New Capacity Providers to Existing Clusters Using CloudFormation

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**Tell us about your request**
I would like to be able to assign a **Capacity Provider** to an existing **ECS Cluster** using CloudFormation.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I have an existing ECS Cluster created with a CFN template. This Cluster will have Services with specific Capacity Providers created with different CFN templates on a new stack. Currently assigning a Capacity Provider to a Service is not enough for the Cluster to 'know' it has a new Capacity Provider, and Services created as such will fail to deploy their tasks due to lack of ECS Instances.

**Are you currently working around this issue?**
I have implemented a lambda function with a custom resource that calls ```putClusterCapacityProviders``` in order to register the new Capacity Providers with the Cluster.

Having to implement this Custom Resource seems unnecessary because if a Capacity Provider is associated with a Service it should automatically be associated with the underlying Cluster. Or we should be able to pass a property to the Capacity Provider Resource defining the Cluster where it'll live.

Thanks

No repository files, tests, or entry points are mentioned. Start by reviewing the ECS and CloudFormation capacity-provider behavior described in the issue and the existing custom-resource workaround; done would mean an agreed CloudFormation-supported way to register a new capacity provider on an existing ECS cluster.

Written by the indexing model from the issue text.