feat: add href to IncomingWebxdcNotify event (#6266)

this PR adds the `href` from `update.href` to the IncomingWebxdcNotify
event (DC_EVENT_INCOMING_WEBXDC_NOTIFY in cffi)

purpose is to add a "Start" button to the notifications that allow
starting the app immediately with the given href

src/events/payload.rs

@@ -117,6 +117,9 @@ pub enum EventType {
 
         /// Text to notify.
         text: String,
+
+        /// Link assigned to this notification, if any.
+        href: Option<String>,
     },
 
     /// There is a fresh message. Typically, the user will show an notification