Switch github.com/golang/protobuf to google.golang.org/protobuf

[DRK3]

As part of #3413, the protobuf type was updated from github.com/golang/protobuf to google.golang.org/protobuf. However, there are some places where the older type from github.com/golang/protobuf is still being used (namely, the proto/tink folder). We should completely switch over to the newer type and also update the generate protobuf files in proto/tink)