feat: add npd-prober example showing NPD + NRC integration for node readiness

[santoshkal]

Description

Adds a complete npd-prober example demonstrating how to use Node Problem Detector (NPD)
custom plugins with the Node Readiness Controller (NRC) to manage node taints based on
HTTP/TCP health probes.

• npd-prober binary: Lightweight probe binary using kubelet-style HTTP (200-399 = healthy)
  and TCP semantics, designed as an NPD custom plugin. Includes --allow-non-local-redirects
  flag matching kubelet behavior, and structured logging via klog/v2.
• NPD integration: ConfigMap-based NPD config that invokes the prober at intervals and
  maps exit codes to a ServiceReadiness node condition.
• NodeReadinessRule: Watches the NPD-managed condition and applies/removes a readiness.k8s.io/ServiceReady=pending:NoSchedule taint based on probe health.
• Kind testing guide: Step-by-step instructions to deploy and verify the full
  NPD → condition → NRC → taint pipeline on a local Kind cluster, including failure
  simulation via service port misconfiguration.

Documents NPD's problem-oriented condition semantics (exit 0 = condition False, exit 1 =
condition True) and how this maps to requiredStatus: "False" in the NodeReadinessRule.

Related Issue

Fixes #138

Type of Change

/kind feature

Testing

Checklist

• go test ./examples/npd-prober/ -v — all unit tests pass
• go vet ./examples/npd-prober/ — no issues
• Follow testing-npd.md on a Kind cluster to verify end-to-end flow

Does this PR introduce a user-facing change?

NONE

Doc #(issue)

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• ✅ login: santoshkal / name: Santosh Kaluskar (b37999d)

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[AvineshTripathi]

I think we should also expand npd-prober to support gRPC, file, and generic exec/command probes as part of this same effort.

Even though Node Problem Detector natively supports executing scripts, pulling all of these standard Kubernetes-style probe methods (http, tcp, grpc, file, exec) into a single, unified npd-prober binary would make the user experience much cleaner and would avoid redundant code examples in the repo.

I believe including these additions now will cover the vast majority of node-readiness-controller + node-problem-detector integration use cases out-of-the-box. What do you think? cc @ajaysundark

p.s. happy to create another issue for this and work on it/even ready to assist