[ECS] [Fargate]: Add custom attributes to Cloudmap via ECS service deploy

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**Tell us about your request**
Enable ECS to add (user provided) custom attributes to Cloudmap

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently it is not possible to tell ECS service to add custom attributes to Cloudmap instance.
[ECS adds certain attributes by default](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/service-discovery.html), but I was thinking of adding additional attributes. eg. LIVE = true, so it allows to do some traffic shaping/simulate deregistration delay etc.. by filtering virtual nodes (in App Mesh) based on its attributes. It can be done with external tooling ofc, but then that needs to be made aware of scale-in and scale-out. eg. the tool needs to update Cloudmap after every task is launched. Would be great if this was supported in ECS by default.

As of now, custom attributes for tasks cannot be specified via task definition or during service creation.

Related Issue on aws-cdk: https://github.com/aws/aws-cdk/issues/1711

**Are you currently working around this issue?**
No

**Additional context**
Migrating to Appmesh

**Attachments**
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Start with the ECS service-discovery documentation linked in the issue and the related aws-cdk issue. Clarify how user-provided attributes would flow through ECS service deployment into Cloud Map and how scaling should preserve them; done means the roadmap has an agreed, implementable scope.

Written by the indexing model from the issue text.