Refactor differ into separate package

Add differ options and package with interface.
Update optional values on diff interface to use options.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

services/diff/service.go

@@ -3,6 +3,7 @@ package diff
 import (
 	diffapi "github.com/containerd/containerd/api/services/diff/v1"
 	"github.com/containerd/containerd/api/types"
+	"github.com/containerd/containerd/diff"
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/plugin"
@@ -39,7 +40,7 @@ func init() {
 			}
 
 			orderedNames := ic.Config.(*config).Order
-			ordered := make([]plugin.Differ, len(orderedNames))
+			ordered := make([]diff.Differ, len(orderedNames))
 			for i, n := range orderedNames {
 				differp, ok := differs[n]
 				if !ok {
@@ -50,7 +51,7 @@ func init() {
 					return nil, errors.Wrapf(err, "could not load required differ due plugin init error: %s", n)
 				}
 
-				ordered[i] = differ.(plugin.Differ)
+				ordered[i] = differ.(diff.Differ)
 			}
 
 			return &service{
@@ -61,7 +62,7 @@ func init() {
 }
 
 type service struct {
-	differs []plugin.Differ
+	differs []diff.Differ
 }
 
 func (s *service) Register(gs *grpc.Server) error {
@@ -102,8 +103,19 @@ func (s *service) Diff(ctx context.Context, dr *diffapi.DiffRequest) (*diffapi.D
 		bMounts = toMounts(dr.Right)
 	)
 
+	var opts []diff.Opt
+	if dr.MediaType != "" {
+		opts = append(opts, diff.WithMediaType(dr.MediaType))
+	}
+	if dr.Ref != "" {
+		opts = append(opts, diff.WithReference(dr.Ref))
+	}
+	if dr.Labels != nil {
+		opts = append(opts, diff.WithLabels(dr.Labels))
+	}
+
 	for _, differ := range s.differs {
-		ocidesc, err = differ.DiffMounts(ctx, aMounts, bMounts, dr.MediaType, dr.Ref)
+		ocidesc, err = differ.DiffMounts(ctx, aMounts, bMounts, opts...)
 		if !errdefs.IsNotImplemented(err) {
 			break
 		}