Improve Envoy Gateway Deployment and Usage

### 🚀 Feature Description and Motivation

+ CP: Deploy Envoy Gateway CP to aibrix-system
+ DP: Enable gateway-namespace mode, which allows envoyproxy deploys in GW CR namespace, preparation for multi-tenant mode.
+ EnvoyPatchPolicy Siimplify: use static backend instead of inject envoy config by hand, which I am working on Envoy Gateway

### Use Case

1. multi-tenant
2. configuration siimplify

### Proposed Solution

_No response_

Start by tracing how Envoy Gateway is deployed for the control plane and data plane, then review the EnvoyPatchPolicy configuration described in the issue. Done means the control plane runs in aibrix-system, gateway-namespace mode supports deployments in the Gateway CR namespace, and EnvoyPatchPolicy uses a static backend instead of hand-injected configuration.

Written by the indexing model from the issue text.