[Fargate] [request]: Support environment files (EnvironmentFile) with Windows containers

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**Tell us about your request**
Support environment files for Windows containers on Fargate, as described in: [Pass environment variables to an Amazon ECS container](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/use-environment-file.html).

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

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

We want to use environment variables inside of Windows containers for non-sensitive configuration values.

Passing a reference to an S3-hosted `.env` file is more convenient than specifying an array of "inline" environment variable objects. This is particularly the case when using CFN, where we try to reuse a single template to create many stacks, and therefore need to "parameterize" aspects that vary among stacks. There is no straightforward means of parameterizing an array of key/value pairs, as is required by [the Environment property](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ecs-taskdefinition-containerdefinition.html#cfn-ecs-taskdefinition-containerdefinition-environment). Frankly, [the EnvironmentFile property](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ecs-taskdefinition-environmentfile.html) has the same issue (it's an array of key/value pairs), but we can get away with parameterizing a reference to a single S3 location, given that single location can contain all needed environment variables (see the example snippet below).

**Are you currently working around this issue?**
We're not yet using environment variables with Windows containers. If we must do so, we'll likely have to create service-specific CFN templates, rather than reusing a common CFN template.

**Additional context**

Though there are 2 existing issues about this (#1626 and #1627), discrepancies remain in the documentation.

Here is a CFN snippet we're using with Linux containers on Fargate, where a reference to an S3-hosted `.env` file is "parameterized".

```YAML
EcsTaskDefinition:
Type: AWS::ECS::TaskDefinition
Properties:
ContainerDefinitions:
- Image: !Ref Image
EnvironmentFiles: !If
- IsEnvironmentFileSpecified
-
-
Type: s3
Value: !Ref EnvironmentFile
- !Ref AWS::NoValue
```

Start by reviewing the linked Amazon ECS environment-file documentation and the related issues #1626 and #1627, focusing on the remaining Windows-container discrepancies. Done means Fargate supports S3-hosted EnvironmentFile references for Windows containers, with the documentation accurately describing the behavior.

Written by the indexing model from the issue text.