feat: Add EventType::CallMissed and emit it for missed calls (#7840)

Before, only `CallEnded` was emitted for missed calls, or, if a call arrives already being stale,
`IncomingMsg`. Now:
- `CallMissed` is emitted in addition to `CallEnded`.
- `IncomingMsg` is replaced with `CallMissed` for stale calls.
Having only one event type for missed calls should simplify handling them in the apps.

This doesn't emit `CallMissed` for those who aren't allowed to call us. Also, don't emit `CallEnded`
if the caller isn't allowed to call us and the call wasn't accepted, as there's no previous
`IncomingCall` event in this case.

src/events/payload.rs

@@ -419,6 +419,14 @@ pub enum EventType {
         chat_id: ChatId,
     },
 
+    /// Call missed.
+    CallMissed {
+        /// ID of the message referring to the call.
+        msg_id: MsgId,
+        /// ID of the chat which the message belongs to.
+        chat_id: ChatId,
+    },
+
     /// One or more transports has changed or another transport is primary now.
     ///
     /// UI should update the list.