outgoing_webhook: Set an X-Smokescreen-Role on requests.

This header is used by the Smokescreen outgoing proxy to provide
identification.

zerver/lib/outgoing_webhook.py

@@ -31,7 +31,12 @@ class OutgoingWebhookServiceInterface(metaclass=abc.ABCMeta):
         self.user_profile: UserProfile = user_profile
         self.service_name: str = service_name
         self.session: Session = Session()
-        self.session.headers.update({"User-Agent": "ZulipOutgoingWebhook/" + ZULIP_VERSION})
+        self.session.headers.update(
+            {
+                "X-Smokescreen-Role": "webhook",
+                "User-Agent": "ZulipOutgoingWebhook/" + ZULIP_VERSION,
+            }
+        )
 
     @abc.abstractmethod
     def make_request(self, base_url: str, event: Dict[str, Any]) -> Optional[Response]: