Gossip traits and persistent storage

- Add `gossip/nostr-gossip` with the gossip traits (like `NostrDatabase`)
- Add `gossip/nostr-gossip-memory` library, which implements the in-memory gossip storage
- Add `gossip/nostr-gossip-sqlite` library, which implements the persistent SQLite gossip storage (ideally supports also `wasm32-unknown-unknown) 