Automatic merge from submit-queue (batch tested with PRs 49342, 50581, 50777)
Device Plugin Protobuf API
**What this PR does / why we need it:**
This implements the Device Plugin API
- Design document: kubernetes/community#695
- PR tracking: [kubernetes/features#368](https://github.com/kubernetes/features/issues/368#issuecomment-321625420)
Special notes for your reviewer:
First proposal submitted to the community repo, please advise if something's not right with the format or procedure, etc.
@vishh @derekwaynecarr
**Release note:**
```
NONE
```
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