message deletion request API (#6576)

this PR adds an API allowing users to delete their messages on other
member's devices

this PR is build on top of
https://github.com/deltachat/deltachat-core-rust/pull/6573 which should
be merged first

a test is missing, otherwise ready for review; it is working already in
https://github.com/deltachat/deltachat-ios/pull/2611

src/headerdef.rs

@@ -80,6 +80,9 @@ pub enum HeaderDef {
     ChatDispositionNotificationTo,
     ChatWebrtcRoom,
 
+    /// This message deletes the messages listed in the value by rfc724_mid.
+    ChatDelete,
+
     /// This message obsoletes the text of the message defined here by rfc724_mid.
     ChatEdit,