Add preemmptable attribute into PodGroup

[k82cn]

What would you like to be added:

Add Preemptable attribute into PodGroup to enable preempt/reclaim action.

Why is this needed:

Currently, we have preemptable annotation for Pod; and it's better to enable Preemptable for PodGroup:

• If PodGroup is preemptable, its Pods can be preemptable or not
• If PodGroup is un-preemptable, its Pods can not be preemptable, we should check this in webhook

[k82cn]

xref: #2105

[stale]

Hello 👋 Looks like there was no activity on this issue for last 90 days.
Do you mind updating us on the status? Is this still reproducible or needed? If yes, just comment on this PR or push a commit. Thanks! 🤗
If there will be no activity for 60 days, this issue will be closed (we can always reopen an issue if we need!).

[stale]

Closing for now as there was no activity for last 60 days after marked as stale, let us know if you need this to be reopened! 🤗

[lowang-bh]

/reopen

[volcano-sh-bot]

@lowang-bh: Reopened this issue.

In response to this:

/reopen

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[ipsum-0320]

/assign