[ECR] [Feature Request]: Support --registry-id flag to retrieve the cross-account image scan findings in describe-image-scan-findings command

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

The ```--registry-id``` flag in the [describe-image-scan-findings](https://docs.aws.amazon.com/cli/latest/reference/ecr/describe-image-scan-findings.html) command specifies the AWS account ID for the target registry. However, when trying to describe the scan findings for cross-account ECR repository images, the command fails with "ScanNotFoundException" due to limitations in Amazon Inspector's integration with ECR enhanced scanning.

It would be good to support the ```--registry-id``` flag to allow the retrieval of image scan findings for cross-account ECR repository images directly.

**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?**
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.
Access image scan findings for images in cross account ECR repository directly which is not possible due to limitation from Amazon Inspector end.

**Are you currently working around this issue?**
How are you currently solving this problem?
The current workaround is to make the Source Account as the [Delegated administrator](https://docs.aws.amazon.com/inspector/latest/user/admin-member-relationship.html) for the Inspector Service or assume an IAM Role/User in the Target Account to be able to describe the image scan findings.

**Additional context**
Anything else we should know?

**Attachments**
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Start with the AWS CLI describe-image-scan-findings documentation and the linked Amazon Inspector cross-account integration guidance. Verify how --registry-id is expected to apply to cross-account ECR images and compare that with the reported ScanNotFoundException. Done means findings can be retrieved directly for a target registry without the stated account or role workaround.

Written by the indexing model from the issue text.