Support UnixFS semantics #1
Description
currently the data bytes of the underlying dagpb node are exposed through this ADL.
It does not however go deeper into interpreting that data based on it's internal protobuf semantics defined at https://github.com/ipfs/go-unixfs/blob/master/pb/unixfs.proto
This is needed to transition FSNode to backed by our data rather than a separate protobuf.