Automatic merge from submit-queue
Add HostAliases to PodSpec to support adding entires to a Pod's hosts file
**What this PR does / why we need it**:
Adds a new field to PodSpec `HostAliases ` to support adding entries to a Pod's hosts file. A PR to incorporate this logic into [`ensureHostsFile`](2899f47bc8/pkg/kubelet/kubelet_pods.go (L208-L227)) in kubelet will be next in order to isolate the discussion on the API.
**Which issue this PR fixes**:
A step into fixing #43632
**Special notes for your reviewer**:
- The API was suggested in https://github.com/kubernetes/kubernetes/issues/43632#issuecomment-293471293
- Concerns with compatible with `hostNetwork: true` Pods are addressed with an validation. https://github.com/kubernetes/kubernetes/issues/43632#issuecomment-293750410
**Release note**:
```release-note
A new field `hostAliases` has been added to `pod.spec` to support adding entries to a Pod's /etc/hosts file.
```
**Testing done**:
- Unit tests pass for validations
- Ran all automated updates, and compared against another PR which added a field in PodSpec to make sure the PR contains all the updated generated code
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