feat(virtual-scroll): Allow non fixed-sized containers

#### Feature Description
When using virtual scroll the element needs to have a set height, or else, by default it'll be 0. It would be really useful for it to be able to have an automatic height.

#### Use Case
For example, i've got a page with aprox. 37 accordions, each with something between 5 and 15 mat-buttons inside each. When opening one, the opening animation is choppy because of how many are rendered on screen (this bug has already been reported in #8287).
I think this can be mitigated in this use case by only rendering what's visible with a virtual-scroll, but, because the height is dynamic(as, when an accordion needs to open, the size of the containing element changes), i cant give it a specific height, thus i can' use

まず、issue に記載された cdk-virtual-scroll-viewport の動作と、#8287 の関連するパフォーマンスの背景を調査します。固定の高さを持たない viewport が、accordion のコンテンツが展開されたときにどのように応答すべきかを判断します。動的な高さのコンテナで、途切れのない開閉アニメーションとともに Virtual Scrolling を使用できれば完了です。

索引モデルが issue の本文から書いたものです。