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/mimefactory.rs

@@ -1098,6 +1098,7 @@ impl MimeFactory {
                                 // Autocrypt 1.1.0 specification says that
                                 // `prefer-encrypt` attribute SHOULD NOT be included.
                                 prefer_encrypt: EncryptPreference::NoPreference,
+                                verified: false,
                             }
                             .to_string();