feat: add multi-gateway namespace support with hybrid gateway capabilities

This feature adds comprehensive multi-gateway support to the Envoy Gateway setup script,
enabling flexible deployments that can separate external and internal services with
appropriate security configurations.

Features:
- Configuration file mode (--config) for YAML-based multi-gateway setup
- Namespace isolation for each gateway (improved security and organization)
- Hybrid gateway support (external-only, internal-only, or both)
- Flexible certificate management (Let's Encrypt or self-signed per gateway)
- Route and listener processing for multiple gateways
- Support for multiple MetalLB pools per gateway
- Internal gateways accessible on port 443 (same as external)
- Backward compatibility with legacy single gateway mode

Configuration Format:
The new --config option accepts YAML files defining multiple gateways:
  gateways:
    - name: external-gateway
      namespace: external-gateway
      domain: cloud.example.com
      type: [external]
      metallb_pools:
        external: gateway-api-external
      issuer:
        type: letsencrypt
        email: [email protected]
      routes:
        - keystone
        - nova
        - neutron

Key Improvements:
- Each gateway runs in its own namespace for better isolation
- Routes are automatically created for each gateway type
- Listeners are applied to all gateways
- Support for multiple DNS providers (Cloudflare, Route53, Azure DNS, etc.)
- Comprehensive error handling and validation

Files Added:
- examples/gateway-config.yaml: Comprehensive example configuration
- examples/simple-gateway-config.yaml: Simple setup example
- docs/gateway-setup.md: Complete usage documentation

Files Modified:
- bin/setup-envoy-gateway.sh: Enhanced with multi-gateway support

Backward Compatibility:
- Legacy single gateway mode still works unchanged
- Existing --email, --domain, --challenge options preserved
- Interactive mode still available
- All DNS plugins supported in both modes

Author: japerezjr

base-kustomize/envoyproxy-gateway/base/envoy-internal-gateway-issuer.yaml

@@ -1,3 +1,4 @@
+---
 apiVersion: cert-manager.io/v1
 kind: ClusterIssuer
 metadata:

base-kustomize/envoyproxy-gateway/base/envoy-service-monitor.yaml

@@ -1,3 +1,4 @@
+---
 apiVersion: monitoring.coreos.com/v1
 kind: ServiceMonitor
 metadata:

base-kustomize/envoyproxy-gateway/base/kustomization.yaml

@@ -1,3 +1,4 @@
+---
 sortOptions:
   order: fifo
 resources:
@@ -8,4 +9,3 @@ resources:
   - envoy-gateway.yaml
   - envoy-endpoint-policies.yaml
   - envoy-service-monitor.yaml
-  - all.yaml