[ECR] [image manifest]: Allow all fields in the image manifest spec

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**Tell us about your request**
Support all fields in [the image spec](https://github.com/distribution/distribution/blob/v2.8.3/docs/spec/manifest-v2-2.md)

**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?**
We have manifest lists that require use of currently unsupported fields in the spec. Mainly, we are trying to use the manifest.platform.features field and our manifests are rejected if it is present. We cannot modify the manifest as it will change the sha of the image. Our platform pulls images with the digest, therefore it won't work if the sha is different after we copy it to ECR.

**Are you currently working around this issue?**
No workaround with ECR. We are using harbor until this issue is solved.

**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.)

Review the linked image specification, focusing on manifest.platform.features, and compare it with ECR's rejection behavior. Done means ECR accepts manifests containing all supported specification fields without changing their digest; the issue names no repository files or tests, so implementation entry points are unspecified.

Written by the indexing model from the issue text.