Add prefetch annotation to converted layers

[wswsmao]

This change adds a boolean layer annotation containerd.io/snapshot/stargz/prefetch to images converted to
eStargz and zstd:chunked. The value reflects whether the layer enables prefetch via stargz landmarks, so users
can inspect it (e.g., via skopeo) without pulling or unpacking the image.

like this:

            "Annotations": {
                "containerd.io/snapshot/stargz/prefetch": "true",
                "containerd.io/snapshot/stargz/toc.digest": "sha256:be4f09591adc9f5e3cb4de2ade94cf2ff086c6abf8007fde0d750ed8b54f906e",
                "io.containers.estargz.uncompressed-size": "41949696"
            }