Add reader.v5.tasks.Task.updated_at.

There was no way to "quickly" get the update date.

dump/src/reader/v5/tasks.rs

@@ -179,6 +179,14 @@ impl Task {
             _ => None,
         }
     }
+
+
+    pub fn updated_at(&self) -> Option<OffsetDateTime> {
+        match self.events.last() {
+            Some(TaskEvent::Created(ts)) => Some(*ts),
+            _ => None,
+        }
+    }
 }
 
 impl IndexUid {