Implement gateway events

[nicoloboschi]

Fixes #352

• Moved the producer creation before the handshake response in the producer endpoint
• Added a new event for client connected and disconnected
• Added new field ´events-topic´ in the gateway which is the topic to write on the events

The write of the client connected event is sync and, in case of failures, will fail the handshake with the client. The event is sent BEFORE the handshake response to the client.

The event message is a JSON containing only the value (no key/headers). An example message is:

{
   "category":"Gateway",
   "type":"ClientConnected",
   "timestamp":1694004909662,
   "source":{
      "tenant":"tenant1",
      "applicationId":"application1",
      "gateway":{
         "events-topic":"topic2",
         "id":"consume",
         "type":"consume",
         "topic":"topic2",
         "authentication":null,
         "parameters":[
            "p"
         ],
         "produceOptions":null,
         "consumeOptions":null
      }
   },
   "data":{
      "userParameters":{
         "p":"consumer"
      },
      "options":{
      },
      "httpRequestHeaders":null
   }

[eolivelli]

Overall LGTM

but I would move the api classes to ai.lanstream.api.events

[eolivelli]

This should be

ai.langstream.api.events