Ability to add child teams

As title says 🙂

It would be advantageous if this bot could support nesting Teams.

The team sync yaml could be enhanced to support something like the following:

```yml
# This property is used to control the addition of general Members to your Organization.
OrganizationMembersGroup: Some_Group_To_Sync_For_Organization_Membership
# This property is used for syncing all other GitHub Teams. Please note that users must also be a part of the `OrganizationMembersGroup` for the synchronization of the teams below to function properly.
GitHubTeamNames:
- Team: Some_Team_To_Sync
FollowChildren: true
- Team: Some_Other_Team_To_Sync
Children:
- Team: Some_Child_Team_To_Sync
Children: Some_Child_Team_To_Sync
- Some_Other_Child_Team_To_Sync
```

Issue にはソースファイル、テスト、エントリポイントが記載されていません。まず、チーム同期の YAML 設定と GitHubTeamNames を処理するコードを見つけ、ネストされた Children と FollowChildren の動作を定義してから、ネストされたチームが意図したとおりに同期されることを確認してください。

索引モデルが issue の本文から書いたものです。