feat(virtual-scroll): Add dynamic fixed size virtual scroll strategy

[Harpush]

Feature Description

Currently the provided virtual scroll strategy is only for fixed size. This is very restrictive.
In cdk experimental there is an autoSize strategy that determines the size of an item automatically but it is experimental and decides the size by itself.
I propose a new strategy for fixed, known but dynamic size.
For example if I have a list of items with some items being with 50px height and some with 80px height I can't use any default strategy for it but I also don't need autoSize. I know item 3 is 80px but item 6 is 50px which makes it easier to implement.
Using the new strategy I can express the dynamic (but known) size.
An example API for it is passing a function: (index: number) => number that returns a pixel size based on given item index.
One of the benefits is that fixed size is a specific case of the new strategy with the function returning the same size always (with a bit of worse performance) - so no need to create two default strategies as fixed (the current one) can be derived from the new strategy.
I created a stackblitz with such strategy and some use cases:
https://stackblitz.com/edit/stackblitz-starters-eey1ve9m

Use Case

1. Can be used for fixed size like now
2. Can be used to add gap between items
3. Can be used to add padding (top/bottom) - although there are other ways to do it currently
4. Can be used to express varying sizes (80px and 50px mixed for example)
5. Can be used to allow expand/collapse items for details - initial size for index X is 50px. On click increase size to 80px and refresh strategy. Same for collapse but the other way around.
6. Can be a mix of all the above

To summarize it gives a lot of control for many use cases including the fixed size that exists today.