Get item_id after voice upload

[luizpvas]

Hello there! First of all, thanks for this amazing library.

From my tests, it seems that the response after voice upload is coming in this format:

{
  message_metadata: [
    {
      thread_id: '<thread_id>',
      item_id: '<item_id>',
      participant_ids: [Array],
      timestamp: '1612822714120679',
      client_context: 'aac90301-da47-53e4-a232-f75590976508'
    }
  ],
  upload_id: '1612822710696',
  status: 'ok'
}

Maybe this is something particular to the account I'm using, or maybe it requires more checks than just reading the first element of message_metadata - the intent of this pull request is just to raise the discussion.

Thanks!