Add Metadata Layer to Pickled Data

[florian-vuillemot]

Description

Currently, pickles are uploaded without an internal metadata layer, which may cause compatibility issues in the future.

To address this, a metadata layer should be added to each stored pickle, as in the encryption module.

By incorporating this metadata, cshelve will support:

1. Backward Compatibility: The ability to parse existing storage to determine whether it adheres to the current cshelve format or an older one.
2. Seamless Migrations: Allows future enhancements (e.g., applying encryption or compression) without requiring service interruptions.

Acceptance Criteria

1. Legacy pickles without the metadata layer are detected and handled without breaking existing projects.
2. Migration scripts can leverage the metadata layer for seamless upgrades.
3. Ensure the metadata is designed to support incremental changes (e.g., versioning for future enhancements).