Remove gogoproto.customtype

gogoproto.customtype is used to have go-digest.Digest instead of string.
While it is convinient, protoc-gen-go doesn't support the extension
and that blocks #6564.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

services/images/helpers.go

@@ -20,6 +20,7 @@ import (
 	imagesapi "github.com/containerd/containerd/api/services/images/v1"
 	"github.com/containerd/containerd/api/types"
 	"github.com/containerd/containerd/images"
+	"github.com/opencontainers/go-digest"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
@@ -58,7 +59,7 @@ func descFromProto(desc *types.Descriptor) ocispec.Descriptor {
 	return ocispec.Descriptor{
 		MediaType:   desc.MediaType,
 		Size:        desc.Size_,
-		Digest:      desc.Digest,
+		Digest:      digest.Digest(desc.Digest),
 		Annotations: desc.Annotations,
 	}
 }
@@ -67,7 +68,7 @@ func descToProto(desc *ocispec.Descriptor) types.Descriptor {
 	return types.Descriptor{
 		MediaType:   desc.MediaType,
 		Size_:       desc.Size,
-		Digest:      desc.Digest,
+		Digest:      desc.Digest.String(),
 		Annotations: desc.Annotations,
 	}
 }