feat: add gateway connectivity status fields to WorkerSliceGateway

[Shreesha001]

Adds fields to the WorkerSliceGateway type consumed by kubeslice-controller and
worker-operator for the Hub-and-Spoke (partial mesh) work:

• Spec.RouteEntireSliceSubnet (bool) — tells the worker to route the entire slice
  subnet via this gateway, so a spoke forwards spoke-to-spoke traffic to the hub.
• Status.ConnectionState / LastTransitionTime / Reason / Message — per-gateway
  connectivity reported by the worker and aggregated into the slice topology status.
• Constants: GatewayConnectionState Connected / NotConnected / Pending.

Additive only (all fields omitempty); no existing field changed.

Fixes #

How Has This Been Tested?

• go build / deepcopy generation (zz_generated) verified.
• Consumed end-to-end on a real 4-cluster KubeSlice setup:
  • the controller sets RouteEntireSliceSubnet on spoke gateways and the worker reads it.
  • spoke-to-spoke iperf traffic relays through the hub.
  • Full mesh verified unaffected.

Checklist

• The title of the PR states what changed and the related issues number.
• Does this PR requires documentation updates?
• I've updated documentation as required by this PR.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have tested it for all user roles.
• I have added all the required unit test cases.

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

No — additive fields only.