[Master feature] Support live AMP Stories

[Enriqe]

Enable the creation of live stories. Users will be able view stories that are updated in real-time as the publisher adds new pages to it - without the need of having the user refresh the page. Think live blogging.

This will unblock use cases like:

• Live sport match reporting (updated scores, commentaries, etc..)
• Live news coverage
• Live keynote updates (e.g. reporter using AMP Stories in keynote event)

We’ll use the amp-live-list functionality to make real-time updates to the DOM. Whenever a new page is detected in the source document, we’ll update the client to reflect the changes. These include a UI treatment and internally letting amp-story know that there is a new page.

This master issue will keep track of the subtasks needed to support this feature.

Subtasks
Title	Description	Status	Github Issue
Modify `amp-live-list` to support dynamically added lists and add configuration options	We will need this to specify `amp-story` as the new container for dynamic elements, see issue for more details	DONE	#21718
Listen to updates at the `amp-story` level	We will need to listen to the `AmpLiveList#update` method, and take the necessary actions when that happens (i.e. show system button UI, insert page in DOM, update progress bar, etc...)	DONE	#21719
Show system UI	Show a UI treatment when there is a new update in the story.	DONE	#21721 #22700
Update `amp-story` progress bar	Whenever a new page is added to the story, we will need to update the progress bar to reflect that. Our current implementation only builds the progress bar at `AmpStory#layoutCallback`	DONE	#21722
Pre-launch testing	Sanity checking and adding more tests	DONE
Launch	Enable validation & submit documentation	DONE

[siddiqrahman]

@Enriqe is it working amp-live-list in amp story?

[Enriqe]

Hi @siddiqrahman, this feature is still in development but you can follow the progress in this issue 👍

[ithinkihaveacat]

FYI there are some issues with amp-live-list getting timely update on some caches, see e.g. the Google internal b/124009333.

[Enriqe]

Thanks for the heads up @ithinkihaveacat 👍

[Enriqe]

Main tasks are now done. Performing sanity checking & closing open discussions. Launching process will follow soon after this.

[Enriqe]

Note there is a problem with amp-live-list getting timely updates in the amp cache. #13659

/cc @newmuis