Automatically generate bundles when repositories are cloned

Currently, bundle uris only work when bundles are created by calling gitaly-backup. This is a manual step that must be configured separately.

Stepping back, we would want to generate bundles only for repositories that are getting cloned. Let's automate bundle creation by adding a step in Gitaly whereby on a clone, we check for the latest bundle and if none exists, we kick off a background task to generate one.

This idea is from @f_santos