Implement a flying carpet visual effect for AMP - applicable to any amp-element in general but specifically for Ads

[kashyapnitin]

Primary objective:
Allow publishers to be able to show as flying carpets
Illustrative (but not necessarily final mock) here

Secondary objective:
Make it really easy in AMP to give the same visual treatment

Rationale:

• It’s a nice visual format
• It’s traditionally not easy to do and would require special treatments / JS in the creative itself
• AMP can make it really easy to give this treatment to any amp-element not just ads, and do so in a performance compliant way
• It is also principally not against any of AMP rules around knowing size and placement of amp-elements in advance
• It can be a good differentiator / pull factor for amp pages
• When used for ads it is likely to generate higher viewability and CTRs (To be verified)

Requirements:

• Publisher should be able to specify that they want AMP runtime to render / layout an or any eligible using the flying carpet layout
• Publisher should be able to pass in an any additional information needed to render / layout the given amp-element as a flying carpet - such as max size of the flying carpet, its position in the page and its position relative to the viewport (when the flying carpet is fully visible)
• Clicks, taps and swipes etc. within the flying carpet window should work as expected
• The flying carpet window should never be larger than the device viewport
• The element that is shown within the flying carpet window is always center aligned

[ericlindley-g]

@jridgewell , passing to you (feel free to correct if you're not the primary Eng owner)—do you have a milestone target for this? (will put in "pending..." for now)

[jridgewell]

Closed by #3118.