Explicitly selected channels for channel controls in Demand Gen ad groups.

[Proto definition ![GitHub](https://developers.google.com/site-assets/logo-github.svg)](https://github.com/googleapis/googleapis/blob/master/google/ads/googleads/v24/resources/ad_group.proto)

| Fields ||
|---|---|
| ## `discover` | `bool` Whether to enable ads on the Discover channel. |
| ## `display` | `bool` Whether to enable ads on the Display channel. |
| ## `gmail` | `bool` Whether to enable ads on the Gmail channel. |
| ## `maps` | `bool` Whether to enable ads on the Maps channel. |
| ## `youtube_in_feed` | `bool` Whether to enable ads on the YouTube In-Feed channel. |
| ## `youtube_in_stream` | `bool` Whether to enable ads on the YouTube In-Stream channel. |
| ## `youtube_shorts` | `bool` Whether to enable ads on the YouTube Shorts channel. |