simplify the envoy installation

### 🚀 Feature Description and Motivation

Follow up of https://github.com/vllm-project/aibrix/pull/1738

step 1
```
helm install eg oci://docker.io/envoyproxy/gateway-helm --version v1.2.8 -n envoy-gateway-system --create-namespace
```

step 2
```
# patch the configuration to enable EnvoyPatchPolicy, this is super important!
kubectl apply -f - <

Start by reviewing the Helm install command and the ConfigMap manifest in the issue, then check envoyproxy/gateway issue #7458 for the required support. Done means the Envoy Gateway setup uses one installation step instead of the current Helm install plus kubectl patch.

Written by the indexing model from the issue text.