Add tooling to generate delta containers

[cgwalters]

Let's make it ergonomic to generate an ostcontainer that also includes static deltas from N previous commits. When fetching the client code would automatically use those instead of the base layer if possible.

[cgwalters]

I'm removing "container exporting ostree on the wire" from the short term roadmap in this project in favor of focusing on container-native flows, e.g. #69 and container deltas.