Initial cut of external plugin support for NPD

[dims]

xref: #833

As a first step to GPU support, we want to avoid running scripts repeatedly and have something running that NPD can talk to over GRPC. I did some experiments in:
https://github.com/dims/npd-ext

Filing this PR as a first step. I've included a sample examples/external-plugins/gpu-monitor that just runs nvidia-smi.

NOTE that the protos are based off the same handshake that external plugins currently use. So this mechanism is NOT just for GPUs in any way.

Please see docs/external-plugins.md for more info.

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[dims]

/retest

[dims]

cc @wangzhen127 @hakman

[dims]

/retest

[k8s-ci-robot]

@dims: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-npd-e2e-node	49639d2	link	true	/test pull-npd-e2e-node
pull-npd-e2e-kubernetes-gce-ubuntu	49639d2	link	true	/test pull-npd-e2e-kubernetes-gce-ubuntu
pull-npd-e2e-kubernetes-gce-ubuntu-custom-flags	49639d2	link	true	/test pull-npd-e2e-kubernetes-gce-ubuntu-custom-flags
pull-npd-e2e-kubernetes-gce-gci	49639d2	link	true	/test pull-npd-e2e-kubernetes-gce-gci
pull-npd-e2e-kubernetes-gce-gci-custom-flags	49639d2	link	true	/test pull-npd-e2e-kubernetes-gce-gci-custom-flags

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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