[Feature] Control etcd + backup-restore sidecar versions

[rfranzke]

Feature (What you would like to be added):
The etcd-druid should control the both the versions for etcd + backup-restore sidecar.

Motivation (Why is this needed?):
It controls the manifests + configuration for the statefulset and the used versions must fit to it. Hence, it makes sense to control them.

Approach/Hint to the implement solution (optional):
Please use the image vector approach (https://github.com/gardener/gardener/blob/master/charts/images.yaml) with the use of https://github.com/gardener/gardener/tree/master/pkg/utils/imagevector.
It must be possible to overwrite the image vector during deployment time.