Retrieve directory-bound dossier from disk

[Wortex17]

Add simple API call that retrieves the dossier for a specific topic (directory) from disk.
All of this should be wrapped in an FS-specific implementation, to allow different dossier sources in the future.

• If the topic (directory) does not exist, the response should be empty.
• If the dossier does not exist, the response should be an empty dossier with a flag to show it's new
• If the dossier exists, the response should contain the dossier in its fetched state