Add node taint config

[wenxuan0923]

Allows flex node operators to specify taints in the agent config alongside existing labels. Taints are applied at node registration time so the node is properly tainted from the moment it joins the cluster.

[Copilot]

Pull request overview

This PR adds support for configuring Kubernetes node taints in the agent config and ensures they’re applied at kubelet registration time via --register-with-taints, so nodes join the cluster already tainted.

Changes:

• Add node.taints to the agent config model and ensure it is deep-copied safely.
• Plumb taints through bootstrapper → kubelet start action spec and into the systemd kubelet drop-in template.
• Extend the kubelet component proto (StartKubeletServiceSpec) with register_with_taints.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/config/structs.go	Adds NodeConfig.Taints config field and documents expected taint string format.
pkg/config/copy.go	Updates Config.DeepCopy() to clone the taints slice to avoid shared backing arrays.
pkg/config/copy_test.go	Adds test coverage to ensure Node.Taints is not shared between original and copied config.
pkg/bootstrapper/components.go	Plumbs config taints into kubelet start spec and ensure-machine spec via defensive slice copy.
components/kubelet/v20260301/kubelet_service.go	Renders --register-with-taints into kubelet systemd drop-in parameters.
components/kubelet/v20260301/assets/20-flex-node-node-config.conf	Adds conditional --register-with-taints=... to kubelet env args.
components/kubelet/action.proto	Adds repeated string register_with_taints = 5 to StartKubeletServiceSpec.
components/kubelet/action.pb.go	Regenerates Go bindings to include the new proto field.

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