runtime/internal/flow: Add a skelleton for the VC.
runtime/internal/flow: Add a skelleton for the VC.

This only adds an interface and empty implementation.

Change-Id: I8a57268b9012dca22c41264149841b3415eaf31d

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref