This provides a basic implementation for setting a stage1 on a per-pod
basis via an annotation.
It's possible this feature should be gated behind additional knobs, such
as a kubelet flag to filter allowed stage1s, or a check akin to what
priviliged gets in the apiserver.
Currently, it checks `AllowPrivileged`, as a means to let people disable
this feature, though overloading it as stage1 and privileged isn't
ideal.
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