[ECR] [request]: Ability to enable KMS Encryption on repos after creation

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**Tell us about your request**
With ECR, you can only enable KMS encryption on a repository during its creation. I am proposing for the ability to add KMS encryption after a repository's creation.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I have a customer who wanted to enable KMS Encryption to all the company's repositories. There are hundreds of images per repository and quite a bit of repositories as well. Because ECR doesn't have the ability to add KMS Encryption after the fact, my customer now has to manually create new repositories and transfer over the hundreds of images, all just to add the KMS Encryption feature. This is an inconvenient and time consuming solution for the customer to be able to use our KMS Encryption.

**Are you currently working around this issue?**
The customer will be manually creating new repositories as a workaround.

**Additional context**
Adding the ability or giving customers a more efficient workaround.

**Attachments**

No repository files, entry points, or tests are named. Start by reviewing the ECR repository encryption API and its constraints, then determine whether existing repositories can support changing encryption settings without moving images. Done means ECR provides an approved way to enable KMS encryption after repository creation, or documents an efficient supported workaround.

Written by the indexing model from the issue text.