[ECS] [request]: Support for mutation of task definitions

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**Tell us about your request**

Are there plans to support something similar to [k8s admission webhooks](https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/) in ECS? This would help us be aware of the tasks getting registered in the cluster and opens up a possibility of performing some custom mutations on the task definition.

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

ECS (Both EC2 and Fargate)

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

Trying to add some sidecar containers that will run alongside/before the app container starts.

**Are you currently working around this issue?**
How are you currently solving this problem?

Add those containers manually to the task definition

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

**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.)

Start with the request and the linked Kubernetes admission webhooks documentation; compare the desired behavior with ECS for both EC2 and Fargate. Done means ECS supports custom mutation of registered task definitions, including adding sidecars that run alongside or before the application container.

Written by the indexing model from the issue text.