[ECS] [request]: Support Sidecar Containers on AWS Batch Task

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**Tell us about your request**
Please allow support sidecar containers on AWS Batch Task.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
The lack of sidecar container support for AWS Batch is causing AWS Batch to have issues with its auto-scaling.

**Are you currently working around this issue?**
Users are currently required to run Datadog Agent as a daemon service. With this setup, users are having issues with AWS Batch properly auto-scaling.

**Additional context**
This is for Datadog users.

**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.)

The issue names no repository files, entry points, or tests. Start by reviewing the AWS Batch and ECS sidecar-container requirements described here, including the Datadog Agent daemon-service workaround, and define what support would need to accomplish for AWS Batch autoscaling.

Written by the indexing model from the issue text.