geranos: skip already uploaded images

[BenTheElder]

1. Upload the manifests after uploading the layers, to record that we've already uploaded a given image digest to the bucket (with an added backup of the manifest contents instead of an empty key).
2. Skip re-uploading layers when we already have the manifest uploaded.

DOES NOT serve clients from the mirrored manifests.

Should produce a pretty large speedup on the second run. We'll be able to skip making any calls for these images except 1) bulk listing references in the registry 2) one S3 call per manifest to confirm it's already uploaded.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BenTheElder

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes/registry.k8s.io/blob/main/OWNERS)~~ [BenTheElder] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ameukam]

/lgtm