Allow replacement of S3 Bucket URL with a CDN URL

Summary Some S3-compatible providers, such as Backblaze B2, allow for delivering of files through CDNs where the bucket URL can simply be replaced by the CDN URL. An additional environment variable to allow replacing the bucket URL with a CDN URL would be a great addition. This is briefly mentioned in #7612 , where it was mentioned that a delivering content directly from cloud object storage is difficult to implement, but a URL replacement in this case might be simpler. Basic Example Some S3-compatible providers allow for delivering of files through CDNs. One such example is Backblaze B2, which has [articles]( on how that can be done. If a B2 Bucket is successfully connected to Cloudflare, a file at (CDN URL) will be mapped to (B2 Bucket URL). This remapping seems to be a simple replacement of only the front part of the full URL. It might be possible to allow for an additional environment variable STORAGE_*_CDN to route file retrieval from the CDN instead. If this environmen…