Relates tocoder/internal#910

This PR adds a scaletest runner that simulates users receiving notifications through WebSocket connections.

An instance of this notification runner does the following:

1. Creates a user (optionally with specific roles like owner).
2. Connects to /api/v2/notifications/inbox/watch via WebSocket to receive notifications in real-time.
3. Waits for all other concurrently executing runners (per the DialBarrier WaitGroup) to also connect their websockets.
4. For receiving users: Watches the WebSocket for expected notifications and records delivery latency for each notification type.
5. For regular users: Maintains WebSocket connections to simulate concurrent load while receiving users wait for notifications.
6. Waits on the ReceivingWatchBarrier to coordinate between receiving and regular users.
7. Cleans up the created user after the test completes.

Exposes three prometheus metrics:

1. notification_delivery_latency_seconds - HistogramVec. Labels = {username, notification_type}
2. notification_delivery_errors_total - CounterVec. Labels = {username, action}
3. notification_delivery_missed_total - CounterVec. Labels = {username}

The runner measures end-to-end notification latency from when a notification-triggering event occurs (e.g., user creation/deletion) to when the notification is received by a WebSocket client.