Add revision labels to PodTemplates in order to automatically rotate Deployments

[xrstf]

Summary

This PR adds "revision labels" to the PodTemplates in the Deployments managed by the kcp-operator: One label for each mounted Secret/ConfigMap, containing those objects' resource version.

The usecase is to automatically rotate Deployments whenever Secrets change, which happens every time a Certificate is renewed or changed (like a new organization being added).

The new middleware will return an error if a mounted object does not exist yet, so while a new Shard or RootShard or FrontProxy is being rolled out, a lot of errors would happen until all Certificates have been issued by cert-manager. To avoid spamming the logs, the operator swallows these errors and relies on it watching the Secrets to reconcile again later. The other alternative would be to skip labels for non-existent mounted Secrets, but this will just lead to lots of pointless pods being created (pointless because even though the operator ignores the missing Secrets, Kubernetes cannot and so the Pods would all be in ContainerCreating). This feels worse than holding off the Deployment creation.

What Type of PR Is This?

/kind feature

Release Notes

Add revision labels to PodTemplates in order to automatically rotate Deployments whenever a mounted Secret changes.

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[xrstf]

/kind feature