kubernetes_deployment_startup: add VPA CPU Startup Boost scenario

PR 3 of 3 — Baseline/Optimized Scenario + CPU Startup Boost via VPA.

Completes the 4-configuration benchmark matrix:
  Config 1: GKE  w/o cpuboost (scenario=baseline, cloud=GCP)
  Config 2: AWS  w/o cpuboost (scenario=baseline, cloud=AWS)
  Config 3: AKS  w/o cpuboost (scenario=baseline, cloud=Azure)
  Config 4: GKE  w/  cpuboost (scenario=optimized, cloud=GCP)

Changes:
- kubernetes_deployment_startup_boost_factor flag (default 2).
- kubernetes_deployment_startup_vpa_yaml flag (slowjvmstartup_vpa.yaml.j2).
- CheckPrerequisites(): raises if scenario=optimized on non-GCP or vLLM.
- GetConfig(): sets enable_vpa=True for scenario=optimized.
- Prepare(): applies VPA manifest with cpuBoostFactor for optimized.
- Run(): adds boost_factor to all sample metadata.
- slowjvmstartup_vpa.yaml.j2: VPA with startup boost policy.

Stacked on PR #6865 (metrics) and PR #6867 (vLLM workload).

Author: ashishsunejaepam