refactor: add virtual scroll adapter

[literalpie]

taking over for PR #14287 which has gone a while without updates. Includes changes to address comments

As discussed in #10122

In order to integrate virtual scroll into mat table we need to decouple CdkVirtualScrollViewport from CdkVirtualForOf

This modification introduces an adapter between the 2 classes, so users can provide their own version of
CdkVirtualScrollAdapter CdkVirtualScrollRepeater

suggested by @shlomiassaf,

[sumigoma]

@literalpie any particular reason why you closed this?

[literalpie]

Mostly just second guessing whether I want to own this. I admit I don't totally understand what's going on, so when I saw that the build failed, I wimped out

[CharlBest]

Haha so honest :)

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