[ECS][Request]: create IAM Managed Policy for ECS Exec

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**Tell us about your request**

I would like the ECS Policy defined on the page [Using Amazon ECS Exec for debugging](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-exec.html) to be available as an IAM Managed Policy.

The policy is below as well for reference:

```
{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": [
"ssmmessages:CreateControlChannel",
"ssmmessages:CreateDataChannel",
"ssmmessages:OpenControlChannel",
"ssmmessages:OpenDataChannel"
],
"Resource": "*"
}
]
}
```

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

The problem I'm trying to solve is consistency and versioned updates across all AWS customers. While this is a small amount of effort for AWS to implement in AWS customer ecosystem this is a considerable amount of effort and potential source of errors.

**Are you currently working around this issue?**

We are creating/managing this policy within an AWS CloudFormation template.

**Additional context**

I (personally) feel like AWS should have a majority of permission snippets available as IAM Managed Policies or somehow use a the service property `enable-exec` to enable the required permission behind the scenes.

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Review the linked Amazon ECS Exec documentation and the policy JSON included in the issue, then compare it with the requested IAM Managed Policy outcome. Done means the ECS Exec permissions are available as an AWS-managed policy; no repository files, entry points, or tests are identified.

Written by the indexing model from the issue text.