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/message.rs

@@ -973,6 +973,10 @@ impl Message {
             | SystemMessage::WebxdcStatusUpdate
             | SystemMessage::WebxdcInfoMessage
             | SystemMessage::IrohNodeAddr
+            | SystemMessage::OutgoingCall
+            | SystemMessage::IncomingCall
+            | SystemMessage::CallAccepted
+            | SystemMessage::CallEnded
             | SystemMessage::Unknown => Ok(None),
         }
     }