CDK Virtual scroller with multiple columns

The feature i would like to request is the possibility to choose multi-column layout while using CDK Virtual scroll.

The use-case for this proposal is expansion of cdk virtual scroll functionality for audio/video portals, for SPA's with collections of different data, for every place where the scroller with a grid-layout is necessary.

I have already done something like that by adding flex styles to container with `cdk-virtual-scroll-content-wrapper` class.

Start by examining the CDK virtual scroll behavior around the `cdk-virtual-scroll-content-wrapper` class and the existing flex-style workaround described in the issue. Define how multi-column layouts should work for the stated grid use cases; done means the requested layout is supported without relying on that workaround.

Written by the indexing model from the issue text.