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>

task_opts.go

@@ -70,7 +70,7 @@ func WithTaskCheckpoint(im Image) NewTaskOpts {
 				info.Checkpoint = &types.Descriptor{
 					MediaType:   m.MediaType,
 					Size_:       m.Size,
-					Digest:      m.Digest,
+					Digest:      m.Digest.String(),
 					Annotations: m.Annotations,
 				}
 				return nil