feat: AppendImage/Index with optional name.Reference

[dtrudg]

Adds f.AppendImage and f.AppendIndex, which append a v1.Image or a v1.ImageIndex to the SIF, respectively.

The OptAppendReference functional option can be used to provide a name.Reference that will be stored as an annotation against the new descriptor in the RootIndex, with the key org.opencontainers.image.ref.name, as is convention for associating references with items stored in an OCI layout.

Fixes #83
Fixes #81

Note reference set in golden files:

$ singularity sif dump 7  pkg/sif/testdata/TestAppendImage/WithReference.golden | jq
{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.index.v1+json",
  "manifests": [
    {
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "size": 525,
      "digest": "sha256:432f982638b3aefab73cc58ab28f5c16e96fdb504e8c134fc58dff4bae8bf338",
      "platform": {
        "architecture": "arm64",
        "os": "linux",
        "variant": "v8"
      }
    },
    {
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "size": 423,
      "digest": "sha256:e4ccb8ccf3c6e47f195a31b13bcf7f3a0075d06dabe450d373f2720f72bab898",
      "annotations": {
        "org.opencontainers.image.ref.name": "myimage:v1"
      }
    }
  ]
}

$ singularity sif dump 8  pkg/sif/testdata/TestAppendIndex/WithReference.golden | jq
{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.index.v1+json",
  "manifests": [
    {
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "size": 525,
      "digest": "sha256:432f982638b3aefab73cc58ab28f5c16e96fdb504e8c134fc58dff4bae8bf338",
      "platform": {
        "architecture": "arm64",
        "os": "linux",
        "variant": "v8"
      }
    },
    {
      "mediaType": "application/vnd.oci.image.index.v1+json",
      "size": 250,
      "digest": "sha256:530cfe4c47e21838156cb43fe8e3be6fefcc3c2d8e7f4f575d7075a9b29b2343",
      "annotations": {
        "org.opencontainers.image.ref.name": "myindex:v1"
      }
    }
  ]
}

[tri-adam]

A few comments/questions on this one. Thanks!