Ensure collectLayers resursively fetch layer for nested index

[jonesbusy]

For the moment we assume all ManifestDescriptor are application/vnd.oci.image.manifest.v1+json when we copy them (via collect layer)

From the spec

This descriptor property has additional restrictions for manifests. Implementations MUST support at least the following media types:

[application/vnd.oci.image.manifest.v1+json](https://github.com/opencontainers/image-spec/blob/main/manifest.md)

Also, implementations SHOULD support the following media types:

application/vnd.oci.image.index.v1+json (nested index)

So we SHOULD support also application/vnd.oci.image.index.v1+json

Also we should also ignore other mediaType if encoutered (in the future)