content: change Writer/ReaderAt to take OCI

This change allows implementations to resolve the location of the actual data
using OCI descriptor fields such as MediaType.

No OCI descriptor field is written to the store.

No change on gRPC API.

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

diff/walking/differ.go

@@ -86,7 +86,11 @@ func (s *walkingDiff) Compare(ctx context.Context, lower, upper []mount.Mount, o
 				config.Reference = uniqueRef()
 			}
 
-			cw, err := s.store.Writer(ctx, config.Reference, 0, "")
+			cw, err := s.store.Writer(ctx,
+				content.WithRef(config.Reference),
+				content.WithDescriptor(ocispec.Descriptor{
+					MediaType: config.MediaType, // most contentstore implementations just ignore this
+				}))
 			if err != nil {
 				return errors.Wrap(err, "failed to open writer")
 			}