Add exec credential plugin policy (kuberc credentialPluginPolicy parity)

[Copilot]

Kubeconfig exec credential plugins start local processes. This PR lets applications restrict which plugins can run, using the same rules as kubectl's kuberc credential plugin policy. It ports kubernetes-client/csharp#1930. The default stays AllowAll, so existing behaviour doesn't change.

const kc = new KubeConfig();
kc.loadFromDefault();

kc.setExecCredentialPluginPolicy({
    credentialPluginPolicy: 'Allowlist',
    credentialPluginAllowlist: [{ command: 'kubelogin' }, { command: '/usr/local/bin/my-plugin' }],
});
// or: kc.loadExecCredentialPluginPolicyFromKuberc();   // $KUBERC → ~/.kube/kuberc

Changes

• src/exec_credential_policy.ts (new)
  • Policy types use the same field names as kuberc: credentialPluginPolicy, credentialPluginAllowlist[].command, and the deprecated name alias.
  • validateExecCredentialPluginPolicy follows client-go's ValidatePluginPolicy. These are errors:
    • an unknown policy value
    • an allowlist set when the policy isn't Allowlist
    • a missing or empty allowlist
    • an empty entry, or one that sets both name and command
    • a path that isn't already normalized (e.g. ./foo)
  • checkExecCredentialPluginPolicy follows client-go's checkAllowlistLocked. A plugin is allowed if its command exactly matches an entry, or if both resolve through PATH to the same executable. Symlinks and globs are not resolved. Relative PATH entries are ignored.
  • Blocked plugins throw ExecCredentialPluginDeniedError.
  • parseKubercExecCredentialPluginPolicy reads only the policy fields from a kuberc file (kubectl.config.k8s.io/v1beta1 or v1, kind Preference) and ignores everything else.
• ExecAuth
  • Takes the policy in its constructor or via setPolicy(). It is checked before any process starts, on the first call and on every token refresh.
  • Changing the policy clears cached tokens.
  • In Allowlist mode the resolved full path is executed. Node's spawn uses options.env.PATH for lookup, so without this a kubeconfig env: PATH=... could swap in a different binary with an allowlisted name. Go's exec.Command resolves against the parent's PATH, so this matches its behaviour.
• KubeConfig
  • setExecCredentialPluginPolicy(policy?)
  • loadExecCredentialPluginPolicyFromKuberc(file?):
    • Looks up the file the way kubectl does: the file argument, then $KUBERC, then ~/.kube/kuberc.
    • Skips loading when KUBERC=off or KUBECTL_KUBERC=false.
    • A missing default file is fine; a missing explicitly named file (argument or $KUBERC) is an error.
    • Nothing loads the kuberc file automatically; the application has to call this.
• Exports and docs: the new types and helpers are exported from index.ts, and the README has a new section.

Review notes

• Environment variables (differs from the C# port): C# rejects every environment variable from the kubeconfig in Allowlist mode. kubectl doesn't, so this PR doesn't either. A kubeconfig PATH can't change which binary runs (see above), but other variables such as LD_PRELOAD still reach the plugin, as they do in kubectl. Rejecting them would be stricter but would break parity with kubectl.
• Unresolvable commands: in Allowlist mode a command that can't be found on PATH is refused before spawning, even if its name is listed. kubectl also fails here, just later when it runs the command. Refusing early stops a kubeconfig-supplied PATH from being used for the lookup.
• Scope: only exec plugins are covered, including the authProvider.config.exec form. The cmd-path options of the Azure and GCP auth providers are not, which matches kubectl.

[kubernetes-prow]

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