feat: verify contacts via Autocrypt-Gossip

This mechanism replaces `Chat-Verified` header.
New parameter `_verified=1` in `Autocrypt-Gossip`
header marks that the sender has the gossiped key
verified.

Using `_verified=1` instead of `_verified`
because it is less likely to cause troubles
with existing Autocrypt header parsers.
This is also how https://www.rfc-editor.org/rfc/rfc2045
defines parameter syntax.

src/mimefactory.rs

@@ -1081,6 +1081,17 @@ impl MimeFactory {
                                             .is_none_or(|ts| now >= ts + gossip_period || now < ts)
                                 };
 
+                            let verifier_id: Option<u32> = context
+                                .sql
+                                .query_get_value(
+                                    "SELECT verifier FROM contacts WHERE fingerprint=?",
+                                    (&fingerprint,),
+                                )
+                                .await?;
+
+                            let is_verified =
+                                verifier_id.is_some_and(|verifier_id| verifier_id != 0);
+
                             if !should_do_gossip {
                                 continue;
                             }
@@ -1091,7 +1102,7 @@ impl MimeFactory {
                                 // Autocrypt 1.1.0 specification says that
                                 // `prefer-encrypt` attribute SHOULD NOT be included.
                                 prefer_encrypt: EncryptPreference::NoPreference,
-                                verified: false,
+                                verified: is_verified,
                             }
                             .to_string();