chore: Add kfp-kubernetes to the core dependencies

[kramaranya]

Description of your changes:
Add kfp-kubernetes to the core dependencies, since pipelines using k8s-specific features require kfp-kubernetes to compile.

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[Copilot]

Pull request overview

This PR adds kfp-kubernetes as a core dependency to ensure that pipelines using Kubernetes-specific features can compile without requiring manual installation of the package.

Changes:

• Added kfp-kubernetes>=2.15.2 to the core dependencies list

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