[node] Websockets for event listeners

[rhlsthrm]

Instead of gRPC we can use Websocket based event handlers.

[rhlsthrm]

Task breakdown:

• [ ] Use this plugin in our server node Fastify server: https://github.com/fastify/fastify-websocket
• [ ] Send events for engine nodes through websocket payloads
• [ ] Update RestServerNodeService .on, .once, .waitFor methods to use websockets instead of webhooks
• [ ] Update router to use this interface and use websockets
• [ ] Update tester instances to use this interface