Refactor storage middleware package

Context

The GitLab Container Registry currently supports two storage middleware:

• AWS CloudFront: Inherited from upstream;
• Google Cloud CDN: Implemented in !828 (merged).

Problem

These two middleware components currently share a lot of similar utility methods, mostly for the construction functions and tests.

Solution

Refactor the storage middleware package, extracting common functionality and reusing it across AWS CloudFront and Google Cloud CDN implementations.