Pixi: Implement global cache for API responses

Currently Safe Browsing and Mobile-friendliness are cached in browser `sessionStorage`. These can be moved to a server cache as well as include PSI results.

The implication of caching the responses also includes proxying API requests to the server where they will be cached.

Additional point from @matthiasrohmer:
> this increases the priority for https://github.com/ampproject/amp.dev/issues/4747 as it would introduce load to amp.dev’s infra that we successfully pushed to the client until now. Especially as PSI requests take some time…