I2I: AMP Runtime Self-Hosting

## Summary

Make it possible for publishers to self-host the AMP runtime, without breaking AMP validity.

## Design document

- [Design Document](https://docs.google.com/document/d/1Wv7jlnTlLYgo9b841W88yblcHJjLI7GhdBKF60IWaWc/edit#
)
- [Design Review presentation](https://docs.google.com/presentation/d/1wP4XyZ0I1qvN3TDaDByRoHGrXbEfWDR1FCIPD58wnQA/edit)

## Motivation

The benefits are:

* Independence: AMP runtime and components no longer needs to be loaded from Google servers for AMP First sites.
* Control: AMP First sites can align the roll-out of a new AMP version with their own QA (this won’t help them if pages are served in SERPs). They can also rollback a broken runtime version themselves.
* Performance: faster page load and TTI by avoiding an additional HTTPs connection.

## Launch tracker

* [Launch Tracker (TBD)](https://docs.google.com/spreadsheets/d/1UW_0pvYTgjQ4qxxftqUlTOXeVKSE0DV4OGBp3f7ipzA/edit#gid=0)

/cc @ampproject/wg-approvers @mattwomple @honeybadgerdontcare