feat: Embed NetworkPolicy to extensions

[vicentefb]

Related to #32

This PR introduces an opinionated security for sandboxes by integrating the automated creation of K8s NetworkPolicy resources. The change is introduced in the extensions folder.

This is achieved by extending the SandboxTemplate API and implementing the corresponding lifecycle management logic within the sandboxclaim-controller. These changes ensure that all sandboxes are secure by default with a "deny-all" network posture.

Key Changes

NetworkPolicy Integration:

• The SandboxTemplate CRD has been extended with a spec.networkPolicy section, allowing administrators to define a secure "blueprint" for sandbox networking.

• The sandboxclaim-controller now reconciles these settings, creating a unique, "default-deny" NetworkPolicy for each SandboxClaim.

This implementation directly fulfills key security requirements:

• Restricts pod-to-pod communication: Each sandbox gets an individual policy, ensuring it is isolated from other sandboxes.

• Controls ingress: The policy only allows traffic from trusted sources defined in the template (e.g., an L7XLB).

• Restricts egress: All outgoing traffic is blocked by default, except for essential DNS lookups.

• Blocks API server access: This is a direct result of the default-deny egress posture.

Controller RBAC Updates:

• The ClusterRole for the sandboxclaim-controller has been updated to include the necessary permissions to manage networkpolicies across the cluster.

Unit Tests:

• Added unit tests for the NetworkPolicy translation logic, covering various ingress and egress rule configurations.

[k8s-ci-robot]

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[k8s-ci-robot]

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[barney-s]

Thanks for this. Looks like a good approach.