fix: Ensure WebSockets routinely transfer data (#4367)

Fixes #4351.

coderd/httpapi/websocket.go

@@ -0,0 +1,27 @@
+package httpapi
+
+import (
+	"context"
+	"time"
+
+	"nhooyr.io/websocket"
+)
+
+// Heartbeat loops to ping a WebSocket to keep it alive.
+// Default idle connection timeouts are typically 60 seconds.
+// See: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/application-load-balancers.html#connection-idle-timeout
+func Heartbeat(ctx context.Context, conn *websocket.Conn) {
+	ticker := time.NewTicker(15 * time.Second)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+		}
+		err := conn.Ping(ctx)
+		if err != nil {
+			return
+		}
+	}
+}