feat: add secondary verified key

When a key is gossiped for the contact in a verified chat,
it is stored in the secondary verified key slot.

The messages are then encrypted to the secondary verified key
if they are also encrypted to the contact introducing this secondary key.

Chat-Group-Member-Added no longer updates the verified key.
Verified group recovery only relies on the secondary verified key.

When a message is received from a contact
signed with a secondary verified key,
secondary verified key replaces the primary verified key.
When verified key is changed for the contact
in response to receiving a message
signed with a secondary verified key,
"Setup changed" message is added
to the same chat where the message is received.

src/securejoin.rs

@@ -1026,8 +1026,11 @@ mod tests {
             gossip_key_fingerprint: Some(alice_pubkey.fingerprint()),
             verified_key: None,
             verified_key_fingerprint: None,
-            fingerprint_changed: false,
             verifier: None,
+            secondary_verified_key: None,
+            secondary_verified_key_fingerprint: None,
+            secondary_verifier: None,
+            fingerprint_changed: false,
         };
         peerstate.save_to_db(&bob.ctx.sql).await?;