Support to rm image indexes and soci indexes.

[fujiwara]

https://aws.amazon.com/jp/blogs/news/aws-fargate-enables-faster-container-startup-using-seekable-oci/

Now, ECR supports image index and soci index. But ecrm can not detect these index, so ecrm deletes these index images possibility.

1. List up images(not index) that candidate for deletion.
2. Find image index has tag sha256-{sha256 of image}.
3. Get a manifest of image index by ECR BatchGetImage API.
4. Find sha256 of soci index in the manifest.