feat(main): Implement caching of manifests in CAS

To ensure that registry clients which attempt to pull manifests by
their content hash can interact with Nixery, this change implements
persisting image manifests in the CAS in the same way as image layers.

In combination with the previous refactorings this means that Nixery's
serving flow is now compatible with containerd.

I have verified this locally, but CI currently only runs against
Docker and not containerd, which is something I plan to address in a
subsequent PR.

This fixes #102

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