Add volatile keyword

Following CERT-Polska/karton#167, here's a simple support for the "volatile" keyword.
This gives users the ability to choose what tasks they don't want reported, examples of situations where you'd want that are given in the issue.

karton/mwdb_reporter/mwdb_reporter.py

@@ -521,6 +521,11 @@ def process_blob(self, task: Task) -> None:
         )
 
     def process(self, task: Task) -> None:
+        if task.has_payload("properties"):
+            if task.get_payload("properties").get("volatile"):
+                self.log.info("Not reporting volatile task")
+                return
+
         object_type = task.headers["type"]
         mwdb_object: Optional[MWDBObject]