deploy(sandbox-gateway): add kustomize configs for sandbox-gateway

[chengzhycn]

Ⅰ. Describe what this PR does

Created config/sandbox-gateway/ directory with the following kustomization resources:

• kustomization.yaml - Entry point listing all resources
• deployment.yaml - Envoy proxy deployment with sandbox-gateway filter
• configmap.yaml - Envoy configuration
• service.yaml - ClusterIP service on port 10000
• serviceaccount.yaml - ServiceAccount for gateway pods
• rbac.yaml - ClusterRole and ClusterRoleBinding for sandbox resource access

Ⅱ. Does this pull request fix one issue?

NONE

Ⅲ. Describe how to verify it

kustomize build config/sandbox-gateway | kubectl apply -f -

Ⅳ. Special notes for reviews

[kruise-bot]

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[chengzhycn]

/assign @furykerry

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.90%. Comparing base (ff612f0) to head (3c484f0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   57.97%   57.90%   -0.07%     
==========================================
  Files          99       99              
  Lines        6027     6027              
==========================================
- Hits         3494     3490       -4     
- Misses       2291     2295       +4     
  Partials      242      242              

Flag	Coverage Δ
unittests	57.90% <ø> (-0.07%)	⬇️

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