Add changelog

.changesets/feat_subscription_websocket_heartbeat.md

@@ -0,0 +1,17 @@
+### feat(subscription): add configurable heartbeat for websocket protocol ([Issue #4621](https://github.com/apollographql/router/issues/4621))
+
+Add the ability to enable heartbeat for cases where the subgraph drops idle connections.
+For example, https://netflix.github.io/dgs/
+
+Example of configuration:
+
+```yaml
+subscription:
+  mode:
+    passthrough:
+      all:
+        path: /graphql
+        heartbeat_interval: enable #Optional
+ ```
+
+By [@IvanGoncharov](https://github.com/IvanGoncharov) in https://github.com/apollographql/router/pull/4802