Automatic merge from submit-queue (batch tested with PRs 59463, 59719, 60181, 58283, 59966). 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>.
Set shared PID namespace mode based on PodSpec
**What this PR does / why we need it**: This PR enables pod process namespace sharing as an alpha feature, as described in [Shared PID Namespace Proposal](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/node/pod-pid-namespace.md).
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
WIP #1615
**Special notes for your reviewer**:
/assign @dchen1107
**Release note**:
```release-note
When the `PodShareProcessNamespace` alpha feature is enabled, setting `pod.Spec.ShareProcessNamespace` to `true` will cause a single process namespace to be shared between all containers in a pod.
```
fe0e80e8da