feat(BottomSheet): Allow custom container/overlay in which to render the bottom sheet

#### Feature Description

Allow uses to customize where the bottom sheet is attached, other than the body.

#### Use Case

Single page web applications have many components, it would be nice to association the bottom sheet with specific components visually.

This is similar to https://github.com/angular/components/issues/7764. Both components define a viewContainerRef, however that does nothing in regards to bottom sheet placement with in the DOM.

まず Angular Material の bottom-sheet 実装と viewContainerRef の処理を読み、次に配置要件をリンク先の Angular components issue と比較します。アプリケーションが常に bottom sheet を body にアタッチするのではなく、カスタムのコンテナと overlay を選択できるようになれば完了です。

索引モデルが issue の本文から書いたものです。