move artifact package over

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

pkg/cmd/attestation/artifact/file.go

@@ -0,0 +1,25 @@
+package artifact
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/github/gh-attestation/pkg/artifact/digest"
+)
+
+func digestLocalFileArtifact(filename, digestAlg string) (*DigestedArtifact, error) {
+	data, err := os.Open(filename)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get open local artifact: %w", err)
+	}
+	defer data.Close()
+	digest, err := digest.CalculateDigestWithAlgorithm(data, digestAlg)
+	if err != nil {
+		return nil, fmt.Errorf("failed to calculate local artifact digest: %w", err)
+	}
+	return &DigestedArtifact{
+		URL:       fmt.Sprintf("file://%s", filename),
+		digest:    digest,
+		digestAlg: digestAlg,
+	}, nil
+}