List newsletter messages

curl --request GET \
  --url https://api.wazapin.com/v1/newsletters/{newsletterJID}/messages \
  --header 'Authorization: Bearer <token>'

{
  "data": {
    "messages": [
      {
        "message_id": "<string>",
        "newsletter_id": "<string>",
        "timestamp": 123,
        "body": "<string>",
        "type": "<string>"
      }
    ],
    "next_cursor": "<string>"
  }
}