Add support for extra attributes in events

This MR adds support for custom extra attributes in Canopsis events.

One configures connector media, sets one or more extra_* parameters with desired values (can be static, can use Zabbix macros…).

Example config:

extra_customnumber: 42
extra_eventid: {EVENT.ID}
extra_opdata: {EVENT.OPDATA}

Corresponding output event:

{
  "customnumber":"42",
  "opdata":"not available (0)",
  "eventid":"52",
  "connector":"zabbix-wh",
  "connector_name":"zabbix60",
  "event_type":"check",
  "resource":"Zabbix agent availability",
  "output":"Zabbix agent is not available",
  "component":"127.0.0.1",
  "state":1,
  "tags":{
    "scope":"availability",
    "component":"system",
    "class":"os",
    "target":"linux"
  },
  "author":"root",
  "user_id":"ec63a8dc-3610-4474-abdf-ea5a46c2195a",
  "source_type":"resource"
}

• Zabbix 6.0 version: done
• Zabbix 5.0 version: done

Closes #8 (closed)