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/headerdef.rs

@@ -86,6 +86,7 @@ pub enum HeaderDef {
 
     ChatDispositionNotificationTo,
     ChatWebrtcRoom,
+    ChatWebrtcAccepted,
 
     /// This message deletes the messages listed in the value by rfc724_mid.
     ChatDelete,