[ECR] [request]: Add configurability to ECR lifecycle policies to be able to specifically target image indexes artifacts and image signature artifacts.

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**Tell us about your request**
Add configurability to ECR lifecycle policies to be able to specifically target image indexes artifacts and image signature artifacts.

**Which service(s) is this request for?**
Add configurability to ECR lifecycle policies to be able to specifically target image indexes artifacts and image signature artifacts.

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

We are signing images in our ECR Repository according to this guidance: https://docs.aws.amazon.com/AmazonECR/latest/userguide/image-signing.html

We would like to implement an ECR Lifecycle Policy that only keeps the latest image. However it appears the policy "Only keep 1 images total" does not do what we need because it will attempt to only keep 1 image, signature, or image index.

ECR lifecycle policies will fail sometimes due to a race condition between deleting an image and deleting its corresponding image index. If the lifecycle execution tries to delete the image index before deleting the image, it will fail - so it needs to know to delete the image first.

Here is an example of that error:
```
"lifecycleEventFailureDetails": [
{
"lifecycleEventImage": {
"digest": "sha256:d2bce1a58adcbf52953ebbf0c6db8d1e1df48ac04bb8439407fbc50eeafe87d4",
"tagStatus": "Untagged",
"tagList": [],
"pushedAt": 1746202309561
},
"rulePriority": 1,
"failureCode": "ImageReferencedByManifestList",
"failureReason": "Requested image referenced by manifest list: [sha256:524b569833fa9753c385fc062375b4c65f0d6d677fb962c09e39c653883ee695]"
}
]
```

**Are you currently working around this issue?**

Tried to implement these lifecycle policies, but the race condition can occur and cause errors:

```
{
"rules": [
{
"rulePriority": 1,
"description": "Keep only latest tagged image",
"selection": {
"tagStatus": "tagged",
"tagPrefixList": ["latest"],
"countType": "imageCountMoreThan",
"countNumber": 1
},
"action": {
"type": "expire"
}
},
{
"rulePriority": 2,
"description": "Remove untagged images",
"selection": {
"tagStatus": "untagged",
"countType": "sinceImagePushed",
"countUnit": "days",
"countNumber": 1
},
"action": {
"type": "expire"
}
}
]
}
```

**Additional context**
N/A

**Attachments**
N/A

The issue identifies an AWS ECR lifecycle-policy request but names no repository files, tests, or implementation entry points. Start by reviewing the ECR image-signing guidance and the reported lifecycle failure; completion would require an agreed service-side design for targeting image indexes and signature artifacts.

Written by the indexing model from the issue text.