Automatic merge from submit-queue (batch tested with PRs 47958, 46261, 46667, 47709, 47579)
Add a feature gate for Debug Containers
**What this PR does / why we need it**: Adds a feature gate for Debug Containers feature proposed in kubernetes/community#649. This feature requires changes to the kubelet, apiserver and kubectl which can be decoupled except for the gate.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
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