Change log for unknown mt to debug

This log message shows up in the client's logs. For any media type that
the client doesn't know about it will wind up with a warning log.
Downgrade this to debug since it is more of a development concern.

We encountered this trying to fetch Docker plugins which has a media
type for plugin configs.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

images/image.go

@@ -362,7 +362,7 @@ func Children(ctx context.Context, provider content.Provider, desc ocispec.Descr
 			// childless data types.
 			return nil, nil
 		}
-		log.G(ctx).Warnf("encountered unknown type %v; children may not be fetched", desc.MediaType)
+		log.G(ctx).Debugf("encountered unknown type %v; children may not be fetched", desc.MediaType)
 	}
 
 	return descs, nil