feat: support receiving Autocrypt-Gossip with _verified attribute

This commit is a preparation for
sending Autocrypt-Gossip with `_verified` attribute
instead of `Chat-Verified` header.

src/e2ee.rs

@@ -35,6 +35,7 @@ impl EncryptHelper {
             addr: self.addr.clone(),
             public_key: self.public_key.clone(),
             prefer_encrypt: self.prefer_encrypt,
+            verified: false,
         }
     }