[ECS] [request]: Add ability to update task definition

### Please see https://github.com/aws/aws-sdk/issues/406 for background and discussion.

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### Community Note

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**Tell us about your request**
What do you want us to build?

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

**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.

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

**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 linked background discussion at https://github.com/aws/aws-sdk/issues/406; this issue names no repository files, tests, or implementation entry point. Clarify the intended ECS task-definition update behavior and acceptance criteria before work can be considered complete.

Written by the indexing model from the issue text.