virtual-scroll: design system for configuring via provider

[mmalerba]

We want to design a fairly flexible way for users to configure virtual scrolling in their app. Some things we may want to support:

• Set the default scrolling strategy for the whole app
• Override the scrolling strategy for the whole app (e.g. to disable virtual scrolling)
• Set the default scrolling strategy for a particular component type (e.g. all mat-tables)
• Override the scrolling strategy for a particular component type
• Set the scrolling strategy for a specific component instance
• Allowing the scroll strategy to be swapped at runtime