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

[Goldman90]

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

[jelbourn]

Duplicate of #10113

[Goldman90]

this is not a duplicate of #10113 , as this also happens when the items have a fixed size, but the container doesn't

[angular-robot]

Just a heads up that we kicked off a community voting process for your feature request. There are 20 days until the voting process ends.

Find more details about Angular's feature request process in our documentation.

[angular-robot]

Thank you for submitting your feature request! Looks like during the polling process it didn't collect a sufficient number of votes to move to the next stage.

We want to keep Angular rich and ergonomic and at the same time be mindful about its scope and learning journey. If you think your request could live outside Angular's scope, we'd encourage you to collaborate with the community on publishing it as an open source package.

You can find more details about the feature request process in our documentation.