Serialize pending server components by reference (lazy component) (#20137)

This now means that if a server component suspends, its value becomes a
React.lazy object. I.e. the element that rendered the server component
gets replaced with a lazy node.

As of #19033 lazy objects can be rendered in the node position. This allows
us to suspend at the location of the server component while we're waiting
on its content.

Now server components has the same capabilities as Blocks to progressively
reveal its content.