hostcfg: Backwards-compatible "ospkg_pointer"

See accepted proposal: https://git.glasklar.is/system-transparency/project/documentation/-/blob/main/proposals/2024-01-30-hostcfg-ospkg-pointer.md?ref_type=heads

Note: there's a drafty implementation linked in the proposal. See the early feedback in !149 (closed) when MR:ing the ospkg_pointer stuff separately.