V1: allow lightbox to add its own intersection observer to the scheduler

[dvoytenko]

Partial for #31915.
Partial for #31540.

Key changes:

• A container element, such as amp-lightbox can contribute its own intersection observer to the scheduler.
• unmountAll is used from the new resources-container-helper.js.

TODO:

• Rebase on V1: mount/unmount concept and automatic scheduling #33272
• Rebase on Support non-Bento children of Bento elements #33450 (new resource-container-helper.js)
• Tests

[samouri]

I'm a confused why this PR only adds to amp-lightbox as opposed to also being able to remove things. Is there no pre-existing functionality this replaces?

I.e.: does unmountAllWithin also automatically pause all of its subresources? If so we should be able to remove the pause.

[dvoytenko]

I'm a confused why this PR only adds to amp-lightbox as opposed to also being able to remove things. Is there no pre-existing functionality this replaces?

It both adds and removes the container for amp-lightbox. Few other elements would quickly follow.

I.e.: does unmountAllWithin also automatically pause all of its subresources? If so we should be able to remove the pause.

Correct. Unmount also pauses. However, for now, "pause" is still useful in its own right. A little less useful with async pause, but it's still used and maybe would be used in perpetuity.

[samouri]

When is opt_scroller used or planned to be used?

[dvoytenko]

When is opt_scroller used or planned to be used?

I think all Bento components separate the scroller from the root element to avoid modifying user markup.