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
```