[stage 1] Pubsub cache for Libervia

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Prepare Pubsub implementation in Libervia to cache nodes/items so they can be retrieved offline and in clear text. This is necessary to store and manipulate decrypted items, or to archive message retrieved from activityPub.

Milestone(s)

• Implementation of cache in Libervia backend
• CLI commands or settings to configure it (e.g.: how long data are kept in cache)
• documentation for users
• documentation for developers
• tests (either unit test or e2e)