AMP-CONSENT v2 UI support

[zhouyx]

• Support full-screen API to allow iframe to expand to the fullscreen
• Apply modal when expand to fullscreen
• Provide default placeholder before iframe is ready. (30px)
• Allow pub to apply modal to block content with non full screen iframe
• [Optional] Allow iframe to customize initial height.

[torch2424]

Placeholder loader: https://jsbin.com/yerevoz/1/edit?html,output

We should position this in the center of the footer thing. If later on we want CMPs to change this, we will have them create their own, and just replace this default.

[torch2424]

Add a refactor PR, to make ui actions match the same type/action format.

[torch2424]

Back to initial / exit full screen PR

[zhouyx]

Notes from today's meeting:

1. Transition animation required from init prompt (30vh) to full screen
2. Need an exit-fullscreen API to return from fullscreen back to the init prompt
3. Transition animation required from full screen back to init prompt(30vh)
4. No transition animation required to remove iframe. (optionally we can try fade out)
5. @torch2424 to create an example page with the transitions and test in real devices.

[ampprojectbot]

This is a high priority issue but it hasn't been updated in awhile. @torch2424 Do you have any updates?

[torch2424]

Need to change UI API:

{
type: 'consent-ui',
response: 'some-event'
}

[torch2424]

Finished as of #19841

Continuing in #19969

Opened an issue for the optional item at: #19970