Implement basic auth and GCS rendering.

You use the Blobstore API (so it has to make a tiny RPC to convert the GCS path to a BlobKey, AFAIK).

I think there's a Go example here:
https://cloud.google.com/appengine/docs/go/blobstore/reference#BlobKeyForFile
https://cloud.google.com/appengine/docs/go/blobstore/#Go_Serving_a_blob

In Python it looks like this:
https://github.com/grow/jetway/blob/master/jetway/files/files.py#L92