Automatic merge from submit-queue (batch tested with PRs 52367, 53363, 54989, 54872, 54643). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Lift embedded structure out of ManifestURLHeader field
Related: #53833
```release-note
It is now possible to set multiple manifest url headers via the Kubelet's --manifest-url-header flag. Multiple headers for the same key will be added in the order provided. The ManifestURLHeader field in KubeletConfiguration object (kubeletconfig/v1alpha1) is now a map[string][]string, which facilitates writing JSON and YAML files.
```
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