Process external message

curl --request POST \
  --url http://localhost:3000/api/messaging/external-messages \
  --header 'Content-Type: application/json' \
  --data '{
  "platform": "discord",
  "messageId": "<string>",
  "channelId": "<string>",
  "userId": "<string>",
  "content": "<string>",
  "attachments": [
    {}
  ],
  "metadata": {}
}'

{
  "success": true,
  "data": {
    "messageId": "<string>",
    "response": "<string>"
  }
}