feat(): add hub-and-spoke topology fields to SliceConfig CRD

[Shreesha001]

Adds the API for Hub-and-Spoke topology (Partial Mesh MVP), per the ADR (#300).

• New optional spec.topology field on SliceConfig: mode (FullMesh | HubAndSpoke) and hubs. All non-hub clusters are spokes automatically.
• Validation: mode enum and max 2 hubs in the CRD schema; cross-field rules in the webhook (hub must be a slice member, no duplicates, at least one spoke, single hub for this release).
• Backward compatible: no topology field → existing full-mesh behavior, unchanged.
• API only — gateway creation does not consume the field yet (that is Controller: Compute hub-and-spoke edges and publish worker intents (Partial Mesh MVP) #302).

Fixes #301

How Has This Been Tested?

• 12 table-driven unit tests covering every validation rule and the valid paths.
• Local Kind cluster: valid HubAndSpoke config accepted; each invalid config rejected with a clear error; slices without topology unaffected.

Checklist:

• The title of the PR states what changed and the related issues number.
• I have performed a self-review of my own code.
• I have added all the required unit test cases.

Does this PR introduce a breaking change for other components like worker-operator?

No

[Copilot]

Pull request overview

Adds Hub-and-Spoke (partial mesh MVP) API surface to the SliceConfig CRD and enforces the initial validation rules via the admission webhook, while keeping default behavior backward compatible (no spec.topology → full mesh).

Changes:

• Introduces spec.topology (mode, hubs) in the SliceConfig Go types and CRD OpenAPI schema.
• Adds create/update webhook validation for topology cross-field rules (single hub for this release, hub must be a member, etc.) with unit tests.
• Adds a Hub-and-Spoke sample SliceConfig manifest.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
service/slice_config_webhook_validation.go	Adds validateTopology + Hub-and-Spoke rules and wires validation into create/update paths.
service/slice_config_webhook_validation_test.go	Adds table-driven unit tests covering valid/invalid topology configurations.
config/samples/controller_v1alpha1_sliceconfig_hub_and_spoke.yaml	Adds a sample SliceConfig manifest demonstrating the new spec.topology fields.
config/crd/bases/controller.kubeslice.io_sliceconfigs.yaml	Extends the CRD schema with spec.topology (mode enum, hubs maxItems).
apis/controller/v1alpha1/zz_generated.deepcopy.go	Updates generated deepcopy functions for the new TopologySpec.
apis/controller/v1alpha1/sliceconfig_types.go	Adds TopologySpec / TopologyMode types and integrates them into SliceConfigSpec.

Files not reviewed (1)

• apis/controller/v1alpha1/zz_generated.deepcopy.go: Generated file

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