Unified Backups: Metadata display and validation command

Proposal

When handling backups, either in Portable or Cloud-based format, we should have tools to read the stored metadata a validate (when that makes sense) that the references exists and are valid/consistent.

In the case of Portable Backups, it can be as simple as looking whether resources exist, or if we are using .7z format, rely on the archive verification functionality (to make sure its not corrupt).

In case of the Cloud Backups, check that the referred resources are still present (haven't expired or haven't been manually deleted by the user), to ensure a Backup described by the metadata can still be restored.

We should also look at providing a "human display" version of the data inside it (perhaps a simple summary of what that backup contains, creation data, gitlab version, etc).