objstorage: Reinstate get_stream method for streaming content bytes
That method from objstorage interface was removed in 2c7e4913 but that feature is useful and should help improve the download of large bundles cooked by the vault (which uses an objstorage as bundles cache).
So reinstate it in base ObjStorage
class using a naive implementation that
enables to stream content bytes to an objstorage client.
Objstorage backends can still override the method for a more efficient implementation.
Related to swh-vault#885 (closed)