feat: add Google CDN support

This MR adds support for redirecting blob downloads to Google CDN when using the GCS storage backend. This is implemented by adding a new storage middleware, similar to the one that already exists for AWS CloudFront.

Based on !824 (closed).

Related to https://gitlab.com/gitlab-org/container-registry/-/issues/535.

Although a good portion of the code (mostly the construction function and some tests) are identical to the ones that already exist for the CloudFront middleware implementation, I decided to not extract, refactor and reuse that common functionality in this MR and raised a followup issue for that: #547.