Set shared PID namespace mode based on PodSpec

[verb]

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.

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:

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.

[verb]

/sig node
/kind feature

[derekwaynecarr]

this looks good.

is there an e2e in a follow-on?

[verb]

@derekwaynecarr yes, I'll open a PR for node-e2e in time for 1.10. That'll be tracked in #59554

[dchen1107]

/lgtm

[k8s-ci-robot]

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This pull-request has been approved by: dchen1107, verb

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[k8s-github-robot]

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 here.