[Master feature] Easy fly-in-on-scroll

[ericlindley-g]

Feature description:

amp-position-observer makes it possible to create complex scroll-bound & scroll-triggered animations. However, we want to make some of the most common animations trivially simple to implement. This feature covers the case where arbitrary HTML elements translate slightly across the page in as the user scrolls them into view—both the scroll-bound case and scroll-triggered, time-based case. Developers will be able to configure aspects of the animation like trigger points, timing, and animation curve.

Related issues:
TODO

Related PRs:
/cc @nainar, can you add relevant PRs here?

[ampprojectbot]

This issue doesn't have a category which makes it harder for us to keep track of it. @ericlindley-g Please add an appropriate category.