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

Inizia esaminando il comportamento di cdk-virtual-scroll-viewport descritto nell’issue e il relativo contesto delle prestazioni in #8287. Determina come dovrebbe rispondere un viewport senza un’altezza fissa quando il contenuto di un accordion si espande; il lavoro è completato quando i container ad altezza dinamica possono usare Virtual Scrolling senza animazioni di apertura a scatti.

Scritto dal modello di indicizzazione a partire dal testo della issue.