Support for "offline" update model

As opposed to updating the local ostree repo by pulling directly from the Internet via http(s). This is beneficial when the Internet connection is not available, some other mechanism is used to transfer the new snapshot to the target device (f.ex, some other node in the network fetches the update package and then uses some proprietary network to copy the package to a target device) or for devices that never connect to the Internet (deliver snapshot via external media, like a USB flash drive).

On a server side this requires to pre-generate the delta package:

ostree static-delta generate --from=${origstart} --to=${origend} --inline

On a client side this requires:

ostree static-delta apply-offline repo/deltas/${origstart}/${origend}
ostree admin deploy ..