Provide rake command for migrating dependency proxy blobs/manifests to object storage

Proposal

Add a rake task to migrate dependency proxy blobs/manifests in local storage to object storage ie. gitlab-rake gitlab:dependency_proxy:migrate

Update:

• https://docs.gitlab.com/ee/administration/packages/dependency_proxy.html
• https://docs.gitlab.com/ee/administration/object_storage.html if !77468 (merged) merged