refactor: add virtual scroll repeater interface

[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,

I have created a stackblitz showing an example where this could be used.

[literalpie]

#19570 is a more up-to-date version of this PR

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