add "umoci manifest" subcommand

[cyphar]

For multi-arch images we probably need to have an umoci manifest subcommand that lets you do certain operations like construct intermediate index blobs for multi-arch (or blow up all intermediate index blobs into top-level index.json entries).

This also might finally help us resolve the long-standing issues related to the UX of #298, though I am still struggling to think of a non-interactive interface (we could generate a YAML-like description of the references that opens in $EDITOR and users can then add annotations -- though this interface is far from optimal).