create transfer()

[efiop]

In dvc (or dvc-data) we currently have a transfer() https://github.com/iterative/dvc/blob/main/dvc/data/transfer.py method that handles tranfering objects from one odb to another. We need a simpler version of that for dvc-objects that would not care about Trees (and probably their indexes), so that it could be used both in dvc-data and in other applications (e.g. in run-cache)

This will allow us to reduce the boilerplate and improve, test and benchmark this core functionality in a convenient way.

[efiop]

Fixed by #64