Subject descriptor has a platform object, causing pushing to fail with ECR

[swucfe]

docker pull nginx
nydusify convert --with-referrer --source nginx:latest --target localhost:5000/oci:latest
nydusify check --target localhost:5000/oci:latest
aws ecr put-image --repository-name test-manifest --image-tag latest --image-manifest file://nydus_manifest.json

When pushing manifests generated from the command above with nydusify to ECR, it fails with An error occurred (InvalidParameterException) when calling the PutImage operation: Invalid parameter at 'ImageManifest' failed to satisfy constraint: 'Invalid JSON syntax'. It turns out that, to ECR, it is not expected to have a platform object in the subject descriptor.

Even though it's not wrong to have a platform object in the subject descriptor but is atypical. We are curious if it is intentional for nydusify to add platform into the subject descriptor in an artifact manifest.