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/param.rs

@@ -120,6 +120,9 @@ pub enum Param {
     /// For Messages
     WebrtcRoom = b'V',
 
+    /// For Messages
+    WebrtcAccepted = b'7',
+
     /// For Messages: space-separated list of messaged IDs of forwarded copies.
     ///
     /// This is used when a [crate::message::Message] is in the