Snowflake rendezvous using AMP cache

This ticket is meant to track progress on adding AMP cache fronting into Snowflake broker and client.

This is a followup of https://trac.torproject.org/projects/tor/ticket/25804#comment:25:

It turns out that AppEngine is not the only option for domain fronting with Google. Google also provides a service called ​AMP cache for ​AMP pages. What it basically does is proxying random pages on the Internet and making them load faster (e.g. on Google search results). It requires pages to comply with some format though and also strips invisible content, resizes images, etc. Despite it is being served via different domain names (one per real domain) it is still hosted at Google infrastructure which can be fronted.