Introduce events system backed by streaming topics

[nicoloboschi]

Use case

As application developer, I want to listen to all gateway client new connections. I'd like to consume from a known "events" topic such events and react somehow, for example sending a message to the target topic with a welcome message.

Proposal

Since events would be useful in other scenarios (e.g. message process failed event) let's introduce a abstract event API which will be enhanced in the future. The first implementation would be to register clients opening/closing connections to the gateway.

gateways.yaml

gateways:
  - id: consume-output
    type: consume
    events-topic: events-topic

pipeline.yaml

topics:
  - name: "events-topic"
    creation-mode: create-if-not-exists

The API gateway will check for the app configuration and writes to the events topic if configured to do so.
The schema will be the same for all the events (even non about gateways). It will be in JSON and this format:

{
 "type": "ClientConnectedToGateway"|"ClientDisconnectedToGateway"
 "timestamp": <long timestamp>
  "source": {
     # this changes based on the type
     "gatewayId": "xxx",
     "applicationId": "zz",
     "tenant": "tt"
    },
  "data": { 
        # custom data depending on the event kind
       "params": {...}
       "userAgent": ....
   }
}