feat: add call ringing API (#6650)

this PR adds a "ringing" api that can be used for calls later.

see deltachat.h for details about the API; jsonrpc is left out until
things are settled for the needs of android/iOS

UI using this PR already successfully are
https://github.com/deltachat/deltachat-ios/pull/2638 and
https://github.com/deltachat/deltachat-android/pull/3785 ; the "payload"
passed forth and back is optimised for
https://github.com/deltachat/calls-webapp

---------

Co-authored-by: l <link2xt@testrun.org>

src/events/payload.rs

@@ -376,6 +376,36 @@ pub enum EventType {
     /// This event is emitted from the account whose property changed.
     AccountsItemChanged,
 
+    /// Incoming call.
+    IncomingCall {
+        /// ID of the message referring to the call.
+        msg_id: MsgId,
+        /// User-defined info as passed to place_outgoing_call()
+        place_call_info: String,
+    },
+
+    /// Incoming call accepted.
+    IncomingCallAccepted {
+        /// ID of the message referring to the call.
+        msg_id: MsgId,
+        /// User-defined info as passed to accept_incoming_call()
+        accept_call_info: String,
+    },
+
+    /// Outgoing call accepted.
+    OutgoingCallAccepted {
+        /// ID of the message referring to the call.
+        msg_id: MsgId,
+        /// User-defined info as passed to accept_incoming_call()
+        accept_call_info: String,
+    },
+
+    /// Call ended.
+    CallEnded {
+        /// ID of the message referring to the call.
+        msg_id: MsgId,
+    },
+
     /// Event for using in tests, e.g. as a fence between normally generated events.
     #[cfg(test)]
     Test,