gke-deploy: apply existing Namespace configs

[Haihan-Jiang]

Fixes #873.

gke-deploy apply currently handles Namespace objects specially: it checks whether each Namespace already exists, creates it if missing, and then removes Namespace objects from the normal apply list. That means existing Namespace objects are never applied again, so changes such as new labels in the manifest are silently skipped.

This change keeps the existing create-warning behavior for missing namespaces, but always applies the Namespace manifest. Existing namespaces can then be updated by kubectl apply, matching the behavior users expect from applying the same YAML with kubectl directly. Namespace objects are still excluded from the deployment summary.

Validation:

• go test ./deployer ./core/cluster ./core/resource from gke-deploy
• go test ./... from gke-deploy
• git diff --check