Avoids a GET for Google Cloud storage public_urls

[nikz]

Hiya!

As I understand it, the public_url method on the Storage class is intended to return a URL without requiring a GET to the backend. At the moment this is how it works for AWS, but for Google Storage the directory.files.get call will make an HTTP request.

I've modified this to use the public access URL as per Google's documentation.

I've also added a very simple spec that's similar to the AWS-specific ones.

Hope this is helpful!

Nik

[mshibuya]

Refs. #1683, 8447886

[mshibuya]

Thanks 👍