ref(client): Make prepare_event public

Make prepare_event public so that sentry-cli can use it directly instead of via capture_event. This allows sentry-cli to do the actual transmission, enabling debug logging and non-zero exit codes on transmission failure

sentry-core/src/client.rs

@@ -172,7 +172,8 @@ impl Client {
         integration.as_ref().as_any().downcast_ref()
     }
 
-    fn prepare_event(
+    /// Prepares an event for transmission to sentry.
+    pub fn prepare_event(
         &self,
         mut event: Event<'static>,
         scope: Option<&Scope>,