[EKS] [request]: Argo CD capability notification controller

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

EKS Argo CD capability

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

EKS

**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.

I'd like to migrate a self-managed Argo CD installation to the new EKS capability. But, the comparison to self-managed documentation (https://docs.aws.amazon.com/eks/latest/userguide/argocd-comparison.html) states that the Notifications controller is unsupported. Currently, in my self-managed installation, I have notifications sent to slack when deployments start / succeed / fail.

Will the notifications controller be added to the capability in the future?

**Are you currently working around this issue?**
N/A

**Additional context**
N/A

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