[amp-story-player] Allow customization of end of carousel button

### Description

The end of carousel button (last "next page" button on the last page of the last story) does not communicate well what happens when a user clicks it.

Publishers can use the "tap on the last page hook" (`noNextStory` event) and do their own action (e.g. show their own custom bookend, close the player, rewind current story, go back to first...). But the button would always be the "next" icon.
Publishers need to be able to customize the last story last page button, from the default "next" to whatever they want (close/rewind/bookend/subscribe to my twitter/go buy my tshirt....), so it matches the action on click.

/cc @ampproject/wg-stories

### Additional Context

More background in https://github.com/ampproject/amphtml/issues/33912#issuecomment-824878827

Start by locating the amp-story-player implementation and the handling for the tap on the last page hook (noNextStory), then read the linked issue context for the expected behavior. Done means publishers can customize the final story's final-page button so its appearance matches their action, while the existing hook behavior remains usable.

Written by the indexing model from the issue text.