[amp-story-player] Improve performance on players above the fold

Follow up for https://github.com/ampproject/amphtml/issues/30990

Now that we have improved the scenario of the player below the fold https://github.com/ampproject/amphtml/issues/30990, we should improve the scenario of the player above the fold.

The problem is that currently we load the player right away when loading a page that embeds the player above the fold.

We can improve this by not loading the story initially, but rather show a poster image or similar UX treatment (see below screenshot, UX not final). When the user clicks on the player, then the story will start loading. This will also go nicely with https://github.com/ampproject/amphtml/issues/32524 is implemented.

We should probably enable this as a configurable option, since it can mean a breaking change for existing implementations.

/cc @ampproject/wg-stories