Add EndpointSlice API support for Kubernetes 1.33+

[shivvinaykanswal]

Summary

• Adds :endpoint_slices as a new option for :kubernetes_ip_lookup_mode to use the discovery.k8s.io/v1 EndpointSlice API
• The legacy Endpoints API (api/v1/endpoints) is deprecated in Kubernetes 1.33 — this provides a migration path
• Default remains :endpoints for backward compatibility with older clusters

Usage

config :libcluster,
  topologies: [
    k8s: [
      strategy: Cluster.Strategy.Kubernetes,
      config: [
        kubernetes_ip_lookup_mode: :endpoint_slices,
        kubernetes_node_basename: "myapp",
        kubernetes_selector: "app=myapp",
        kubernetes_namespace: "default"
      ]
    ]
  ]

What changed

• lib/strategy/kubernetes.ex: Added :endpoint_slices lookup mode with the discovery.k8s.io/v1 API path and a response parser for the EndpointSlice format (where IPs are in endpoints[].addresses[] instead of subsets[].addresses[].ip)
• test/kubernetes_test.exs: Added 4 tests covering endpoint_slices with default/cached/hostname/dns modes
• test/fixtures/vcr_cassettes/kubernetes_endpoint_slices.json: VCR cassette for EndpointSlice API responses

Addresses bitwalker#213

Test plan

• All 14 existing + new tests pass (mix test test/kubernetes_test.exs)
• Verify on a Kubernetes 1.33+ cluster with kubernetes_ip_lookup_mode: :endpoint_slices
• Verify backward compatibility — existing :endpoints and :pods modes unchanged

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