move artifact package over

Signed-off-by: Meredith Lancaster <malancas@github.com>

pkg/cmd/attestation/artifact/image.go

@@ -0,0 +1,28 @@
+package artifact
+
+import (
+	"fmt"
+
+	"github.com/distribution/reference"
+	"github.com/github/gh-attestation/pkg/artifact/oci"
+)
+
+func digestContainerImageArtifact(url string, client oci.Client) (*DigestedArtifact, error) {
+	// try to parse the url as a valid registry reference
+	named, err := reference.Parse(url)
+	if err != nil {
+		// cannot be parsed as a registry reference
+		return nil, fmt.Errorf("artifact %s is not a valid registry reference", url)
+	}
+
+	digest, err := client.GetImageDigest(named.String())
+	if err != nil {
+		return nil, err
+	}
+
+	return &DigestedArtifact{
+		URL:       fmt.Sprintf("oci://%s", named.String()),
+		digest:    digest.Hex,
+		digestAlg: digest.Algorithm,
+	}, nil
+}