Create a CLI tool to decrypt a CXP export/backup

[Progdrasil]

This CLI should be useful for both testing purposes and for users who have a backup and want to inspect it. I should take the export archive and either the pre-shared-key or the "importing provider's" private key to decrypt and (optionally) deserialize the format.

For users this can be usefull to recover from a provider that went under and their new provider does not support CXP/CXF yet.

For developers, this can be usefull to diagnose any issues in their implementation for either the protocol or the format.

[Hinton]

Sounds reasonable and should be easy to achieve once we support CXP and not just the format.