Geo: Replicate Dependency Proxy

From #235434 (comment 405921331):

@mkozono yes I believe it should be replicated. Besides reliability, dependency proxy is about saving on network and time by pulling images from GitLab vs DockerHub. If you can't use it on secondary instance that is geographically close to you then the feature loses some value.

the implementation practically depends on &1528 or some part of it

When we decide to do this, apply the Blob replicator strategy issue template to this description.