[madejusz]

Description

Adds support for selecting the CNI of an SKE cluster via network.cni. Exactly one of calico or cilium can be set; the field is immutable (changing it forces a new cluster) and defaults to cilium.

network = {
  cni = {
    cilium = {} # or calico = {}
  }
}

Notes:

• The SDK (ske v1.22.0) only models calico in CNI, so cilium is sent via AdditionalProperties.
• Because the default is cilium, existing clusters running Calico need cni = { calico = {} } set explicitly to avoid a replacement.
• The data source exposes network.cni as read-only.

relates to #1714

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

Validation: go test ./stackit/internal/services/ske/... passed, gofmt -l and go vet ./stackit/internal/services/ske/... clean. Acceptance tests and make lint were not run.