feat_subscription_websocket_heartbeat.md

Subscriptions: Add configurable "heartbeat" to subgraph WebSocket protocol (Issue #4621)

To account for GraphQL Subscription WebSocket implementations (e.g., DGS) which drop idle connections by design, the router adds the ability to configure a heartbeat to keep active connections alive.

An example of configuration:

subscription:
  mode:
    passthrough:
      all:
        path: /graphql
        heartbeat_interval: enable #Optional

By @IvanGoncharov in #4802