Introduce `orb migrate` subcommand to enable forward-migration through breaking versions

[cdata]

We are reaching that point in Noosphere deployment where it would be really nice to be able to forward-migrate existing spheres as various file system layout, protocol data structure and other changes land. Currently this is unexplored territory for us, but one low-hanging fruit could be implementing an orb migrate subcommand that "automatically" brings forward an orb-managed sphere workspace to the latest version whenever it is run within that workspace.