OCI: Add From/ToProto helpers for Descriptor

Helpers to convert from the OCI image specs [Descriptor] to its protobuf
structure for Descriptor and vice-versa appear three times. It seems sane
to just expose this facility in /oci.

Signed-off-by: Danny Canter <danny@dcantah.dev>

oci/spec.go

@@ -21,8 +21,11 @@ import (
 	"path/filepath"
 	"runtime"
 
+	"github.com/opencontainers/go-digest"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/opencontainers/runtime-spec/specs-go"
 
+	"github.com/containerd/containerd/api/types"
 	"github.com/containerd/containerd/containers"
 	"github.com/containerd/containerd/namespaces"
 	"github.com/containerd/containerd/platforms"
@@ -220,3 +223,25 @@ func populateDefaultDarwinSpec(s *Spec) error {
 	}
 	return nil
 }
+
+// DescriptorFromProto converts containerds protobuf [types.Descriptor]
+// to the OCI image specs [ocispec.Descriptor].
+func DescriptorFromProto(d *types.Descriptor) ocispec.Descriptor {
+	return ocispec.Descriptor{
+		MediaType:   d.MediaType,
+		Digest:      digest.Digest(d.Digest),
+		Size:        d.Size,
+		Annotations: d.Annotations,
+	}
+}
+
+// DescriptorToProto converts the OCI image specs [ocispec.Descriptor]
+// to containerds protobuf [types.Descriptor].
+func DescriptorToProto(d ocispec.Descriptor) *types.Descriptor {
+	return &types.Descriptor{
+		MediaType:   d.MediaType,
+		Digest:      d.Digest.String(),
+		Size:        d.Size,
+		Annotations: d.Annotations,
+	}
+}