Allow snippet blobs to be cached by an external storage

Related to gitlab-com/gl-infra/scalability#4 and https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6829.

Spun-off from #193498 (closed).

Snippet raw blob paths doesn't match regular blob paths (*/raw vs */raw/*), so we would need update this function. Other than that, the change would mostly look like !20936 (diffs).