Merge pull request #55848 from MrHohn/custom-pod-dns-api

Automatic merge from submit-queue (batch tested with PRs 55812, 55752, 55447, 55848, 50984). 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>.

Add Custom Pod DNS to Kubernetes API

**What this PR does / why we need it**:
Ref:
- Feature issue: kubernetes/enhancements#504
- Proposal: kubernetes/community#1276

This PR adds the relevant APIs, validation check and the underlying kubelet changes.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #NONE

**Special notes for your reviewer**:
/sig network
@kubernetes/sig-network-api-reviews
/assign @bowei @thockin

**Release note**:

```release-note
Add DNSConfig field to PodSpec and support "None" mode for DNSPolicy (Alpha).
```

Kubernetes-commit: 8a5cf788b0560c176bf219f26e893d97a19de1e2