Enables dependency proxy

What does this MR do?

Values selected to match configuration on VMs:

  ## Dependency proxy (EE only)
  dependency_proxy:
    enabled: true
    # The location where dependency_proxy blobs are stored (default: shared/dependency_proxy).
    storage_path: /var/opt/gitlab/gitlab-rails/shared/dependency_proxy
    object_store:
      enabled: true
      direct_upload: true
      background_upload: false
      proxy_download: false
      remote_directory: "gitlab-gprd-dependency-proxy"
      connection: {"provider":"Google","google_project":"gitlab-production","google_json_key_location":"/etc/gitlab/gcs-creds.json"}

Note that we don't have proxy_download enabled for dependency proxy, because this is the default for omnibus:

gitlab_rails['dependency_proxy_object_store_proxy_download'] = false