[pranathibora14]

Add JSON deletion backup storage and undo audit foundations.

• Add a versioned backup model for package and symbol status, archive hashes, external README content, and incoming deprecation references.
• Add JSON save/read support in the private package-delete-backups container and register the service through existing storage wiring.
• Add the UndoSoftDelete audit action and record, including success/failure, with cloud persistence support.

Stack
Part 1 of 6.
Base: feature/undelete.
Later parts build on this PR; the completed feature will be integrated into dev.
Incremental scope: 16 changed files.

Addresses https://github.com/NuGet/Engineering/issues/6512

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[chabiss]

The Shim will need to be updated