Explore signedURLs caching

Improvement suggestion/feature request from our SREs (@pguinoiseau) on Slack:

Something that could help massively in production would be to add caching for the signed URLs so that we don't abuse the IAM API too much (which has a quota that we'll need to increase either way): https://gitlab.com/gitlab-org/gitlab/-/issues/535524

Internal implementation should use https://github.com/hashicorp/golang-lru as this is currently state-of-the-art for caching in Go.