Extend encryption mechanism to sign a Pubsub item with OX (or any better
fitting algorithm)
Milestone(s)
- Write and propose necessary protoXEP(s) to sign a Pubsub item. The
mechanism should be available both for plain text items (e.g. to
sign a public blog post) and for encrypted items.
- Implement Pubsub item signing in Libervia
- CLI commands to sign an item
- documentation
- tests (either unit test or e2e)