Kk/extract manifest manipulation to utils

[karol-kokoszka]

This PR is used as a proof-of-concept for the idea of sharing the manifest manipulation part with the Scylla Cloud, so that SC can import the manifest dependency and just use it to understand snapshot backup content.

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Please make sure that:

• Code is split to commits that address a single change
• Commit messages are informative
• Commit titles have module prefix
• Commit titles have issue nr. suffix

[Michal-Leszczynski]

I know it's just a playground, but before going in this direction, we should carefully choose only the interesting subset backupspec instead of the whole pkg.