Automatic merge from submit-queue (batch tested with PRs 58716, 59977, 59316, 59884, 60117). 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 ShareProcessNamespace to PodSpec

**What this PR does / why we need it**: This adds an option to the Kubernetes API to configure sharing a process (PID) namespace between the containers in a pod, as described in the [shared pid proposal](https://github.com/verb/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**: 
Questions for API reviewer:
- [ ] Is the documentation sufficient to describe the new option?
- [x] Does the new field better belong in the security context?
- [x] is prepending "alpha" to json/proto fields the right thing to do?

**Release note**:

```release-note
v1.Pod now has a field to configure whether a single process namespace should be shared between all containers in a pod. This feature is in alpha preview.
```