Add support for VideoAdvertisingOptions#get

Example usage:

*Adding a third party ad server id to an existing YouTube video*:

```ruby
  content_owner = Yt::ContentOwner.new refresh_token: '...'
  video_id = 'video_id'
  third_party_video_id = 'third_party_video_id'

  yt_video = Yt::Video.new(id: video_id, auth: content_owner)
  new_ad_formats = Set.new(yt_video.ad_formats).add('third_party').to_a
  yt_video.advertising_options_set.update(ad_formats: new_ad_formats, tp_ad_server_video_id: third_party_video_id)
```