feat: added VAP example to enforce security defaults for podTemplate spec

[vicentefb]

fixes #262 , #261

This PR introduces a VAP example with 16 distinct security controls across all containers, init containers, and ephemeral containers in a Sandbox pod. Key enforcements include:

• Runtime & Scheduling: Mandates runtimeClassName: gvisor and enforces the necessary nodeSelector and tolerations to ensure workloads land on isolated node pools.
• Node & Network Isolation: Strictly prohibits hostNetwork, hostPID, hostIPC, and hostPort usage to prevent node metadata exposure and namespace leaks.
• Identity Isolation: Enforces automountServiceAccountToken: false and explicitly blocks Projected volumes from mounting ServiceAccount tokens or Pod Certificates (ClusterTrustBundle).
• Filesystem & Kernel Hardening: Blocks hostPath volumes, unmasked procMount paths, and custom sysctls.
• Privilege Escalation & Defense in Depth: Blocks privileged containers, requires processes to runAsNonRoot (properly evaluating Pod-level inheritance), strictly mandates dropping ALL Linux capabilities, and prevents any from being added back via capabilities.add.
• DoS Protection: Enforces CPU and Memory resource limits on all containers to prevent noisy-neighbor node starvation.

This PR also includes an integration test (examples/policy/vap/policy_test.go).

Utilizes sigs.k8s.io/controller-runtime/pkg/envtest to spin up a local Kubernetes v1.30 API server.

Verifies the CEL expressions by applying the VAP and intentionally attempting to create Sandboxes with isolated vulnerabilities (e.g., privileged containers, wrong runtime classes, malicious volume mounts).

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[vicentefb]

/test presubmit-agent-sandbox-e2e-test

[mtaufen]

/lgtm

[vicentefb]

/assign @janetkuo

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: aditya-shantanu, mtaufen, vicentefb
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[janetkuo]

A few comments; lgtm otherwise

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.