NEWS: Add entry for axoflow#412

Signed-off-by: Tamás Kosztyu <tamas.kosztyu@axoflow.com>

news/feature-412.md

@@ -0,0 +1,16 @@
+bigquery(), google-pubsub-grpc(): Added service-account() authentication option.
+
+Example usage:
+```
+destination {
+    google-pubsub-grpc(
+        project("test")
+        topic("test")
+        auth(service-account(key ("path_to_service_account_key.json")))
+    );
+};
+```
+
+Note: In contrary to the `http()` destination's similar option,
+we do not need to manually set the audience here as it is
+automatically recognized by the underlying gRPC API.