Add support for creating empty image config

[kipz]

According to the current OCI spec:

Content other than OCI container images MAY be packaged using the image manifest. When this is done, the config.mediaType value MUST be set to a value specific to the artifact type or the empty value.

It's currently possible to set the config MediaType:

img = mutate.ConfigMediaType(img, "application/vnd.oci.empty.v1+json")

But it's not currently possible (as far as I can tell), to create the rest of the descriptor:

{
  "mediaType": "application/vnd.oci.empty.v1+json",
  "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a",
  "size": 2,
  "data": "e30="
}

This PR adds a new mutate function to achieve this:

img = mutate.EmptyConfig(img)

An alternative, and maybe more general approach, would be to expose a mutate function to set the value of Manifest.Config.Data directly, and calculate the size/digest during compute.

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