feat: Make gossip period configurable (#4346)

This is needed to test periodic re-gossiping in existing chats.

Also add a test for verified groups on that even if "member added" message is missed by a device of
newly added member, after re-gossiping Autocrypt keys to the group it successfully learns these keys
and marks other members as verified.

src/context.rs

@@ -754,7 +754,6 @@ impl Context {
                 .await?
                 .to_string(),
         );
-
         res.insert(
             "debug_logging",
             self.get_config_int(Config::DebugLogging).await?.to_string(),
@@ -763,6 +762,10 @@ impl Context {
             "last_msg_id",
             self.get_config_int(Config::LastMsgId).await?.to_string(),
         );
+        res.insert(
+            "gossip_period",
+            self.get_config_int(Config::GossipPeriod).await?.to_string(),
+        );
 
         let elapsed = self.creation_time.elapsed();
         res.insert("uptime", duration_to_str(elapsed.unwrap_or_default()));