Automatically add @font-face for custom whitelisted Google fonts via AMP CDN.

Google Fonts and certain whitelisted fonts are extremely popular and widely used. There are two main ways to load them:

1. via css ``````

2. via @font-face

The CSS option is render blocking and causes slowness. The @font-face option is the preferred and super-fast. However, using @font-face option involves adding the `src` which becomes cumbersome when you are using multitude of fonts.

Proposal:
When AMP pages are hosted on AMP CDN,

1. provide a way to automatically parse the rel="stylesheet".
2. discover the white-listed fonts that are being used.
3. inject the fonts directly from the reference stylesheet as @font-face.
4. Don't count the @font-face included by this approach towards the 50KB CSS limit.

Future:
1. Extend the above method towards all fonts, whitelisted and otherwise.

Advantage:
Font's are render blocking and this approach can speed up things while maintaining UI/UX consistency and with less code.

Update: According to https://github.com/ampproject/amphtml/issues/11249, fonts aren't render blocking, however, lightspeed and PSI both suggest to use @font-face instead of the above link-href approach.

Assuming Lightspeed/PSI is incorrect, should we open a ticket on their side?

The issue names no source files or tests. Start by reviewing AMP CDN handling of stylesheet links and the discussion in issue #11249; define the whitelist, font injection behavior, and CSS-limit accounting before implementation. Done means the agreed behavior is specified and covered for supported fonts.

Written by the indexing model from the issue text.