KEP-5339: define extensions-based per-cluster plugin configuration

[kahirokunn]

• One-line PR description:
  Define extensions-based per-cluster plugin configuration and pass-through to ExecCredential.Spec.Cluster.Config

• Issue link:
  sig-multicluster's ClusterProfile should have a way to support credentials issuance #5339

• Other comments:

  • Add a "Passing plugin configuration via extensions" section under Design Details
  • Clarify that the extensions entry is optional, non-secret, and per-cluster; controller- or environment-specific data must not be placed there
  • Specify library behavior to pass the extension object through to ExecCredential.Spec.Cluster.Config when present
  • Add a SecretReader ClusterProfile example that provides clusterName via extensions
  • Reference for the configuration source:
    • client authentication API (v1) Cluster: config sourced from extensions[client.authentication.k8s.io/exec]:
      https://kubernetes.io/docs/reference/config-api/client-authentication.v1/#client-authentication-k8s-io-v1-Cluster
  • Background: the core implementation landed first and this change formalizes the behavior in the KEP:
    https://github.com/kubernetes-sigs/cluster-inventory-api/pull/21/files#diff-9079632b67f704fe47373dc3fc1cf49db8b421592a06ffb89d74d436a65faf2c

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @kahirokunn. Thanks for your PR.

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