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

Start by examining the cdk-virtual-scroll-viewport behavior described in the issue and the related performance context in #8287. Determine how a viewport without a fixed height should respond when accordion content expands; done means dynamic-height containers can use virtual scrolling without choppy opening animations.

Written by the indexing model from the issue text.