Add co-host by the API

### Introduction
On August 11 Google annouced on a blog post that Google Meet would accept up to 25 co-hosts on a meet. [Blog Post](https://workspaceupdates.googleblog.com/2021/08/add-up-to-25-cohosts-expanded-safety-features-for-meet.html).

### Request
It would be nice to add a flag on the class Google_Service_Calendar_EventAttendee where we could delegate that the user would be a co-host upon Meet creation.

### Concerns
On the blog post, it signs that the organizer must be on the room first and on the Calendar interface, threre is no option for adding co-hosts. I don't know if it would interfere with the API or if requesting it here would be the correct thing. It will probably interfere with the backend Meet engine and this might not be the designed behavior for creating a Meet by the API.

Google_Service_Calendar_EventAttendee とリンク先の Google Meet の告知から始め、その後、Calendar API が Meet の作成時の共同ホスト割り当てを公開しているか確認してください。API でサポートされているフィールドと動作が確認できるか、またはリクエストがサポートされていないという結論を文書化できれば完了です。

索引モデルが issue の本文から書いたものです。