feature(CdkVirtualScrollViewport): allow to scroll to center. method: scrollToIndex

#### Feature Description

Allow scrolling to index and center the position.
method: scrollToIndex
component: CdkVirtualScrollViewport

#### Use Case

Scroll into view and center an item.

ex: see HTML scrollToIndex options:

{
behavior: "auto",
block: "center",
inline: "center",
}

Begin with CdkVirtualScrollViewport and its scrollToIndex method. Inspect how index scrolling currently works, then define completion as allowing the requested item to be positioned in the center, consistent with the proposed scroll options.

Written by the indexing model from the issue text.