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/sync.rs

@@ -71,6 +71,9 @@ pub(crate) enum SyncData {
     DeleteMessages {
         msgs: Vec<String>, // RFC724 id (i.e. "Message-Id" header)
     },
+    RejectIncomingCall {
+        msg: String, // RFC724 id (i.e. "Message-Id" header)
+    },
 }
 
 #[derive(Debug, Serialize, Deserialize)]
@@ -264,6 +267,7 @@ impl Context {
                     SyncData::Config { key, val } => self.sync_config(key, val).await,
                     SyncData::SaveMessage { src, dest } => self.save_message(src, dest).await,
                     SyncData::DeleteMessages { msgs } => self.sync_message_deletion(msgs).await,
+                    SyncData::RejectIncomingCall { msg } => self.sync_call_rejection(msg).await,
                 },
                 SyncDataOrUnknown::Unknown(data) => {
                     warn!(self, "Ignored unknown sync item: {data}.");