Add support for ephemeral containers during upgrades

[johscheuer]

What would you like to be added/changed?

The feature is stable since 1.25: https://kubernetes.io/docs/concepts/workloads/pods/ephemeral-containers. We could add support for this feature and allow a user to decide which upgrade mode they want to use. We should implement this mode only for the unified image, with the sidecar approach the benefit will be limited.