add config option for message_send_checkpoint_endpoint

mautrix_signal/example-config.yaml

@@ -14,6 +14,8 @@ homeserver:
     # If set, the bridge will make POST requests to this URL whenever a user's Signal connection state changes.
     # The bridge will use the appservice as_token to authorize requests.
     status_endpoint: null
+    # Endpoint for reporting per-message status.
+    message_send_checkpoint_endpoint: null
 
 # Application service host/registration related details
 # Changing these values requires regeneration of the registration.