KEP for sandbox warmpool to create sandboxes

[peterzhongyi]

Please see the README for the proposal. This is the first time I'm writing this, so please point out things that I should fix!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: peterzhongyi
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[k8s-ci-robot]

Hi @peterzhongyi. Thanks for your PR.

I'm waiting for a github.com member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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