Serve all /api/... requests needed for `pub get` and `pub upgrade` from statically generated files

[sigurdm]

If we could pregenerate the response to

/api/packages/foo
/api/packages/foo.tar.gz

And store it for example in a cloud storage bucket, and statically redirect all queries directly to that bucket would make it much easier to feel confident in the availability of pub get and pub upgrade.

[jonasfj]

Not quite, we won't want to redirect. We'll want to serve directly from the bucket...

Thus, we'll need a new subdomain like get.pub.dev which hosts this static content.

Note. This is also blocked on popularity metrics being collected by some other means.. hence I think we need a bit of design work first.

[sigurdm]

We still want this!