Enable Kubernetes Service Discovery for API7 Gateway upstreams

Configured API7 Gateway to use Kubernetes Service Discovery instead of
static upstream nodes. This enables dynamic discovery of backend Pods
through the Kubernetes API.

Benefits:
- Automatic scaling: New Pods are automatically added to upstream pool
- Health checks: Only healthy Pods receive traffic
- Zero downtime: Automatic updates during deployments and rollouts
- No manual upstream configuration needed

Changes:
- Updated configmap-adc.yaml to use discovery_type: kubernetes
- Service discovery queries Kubernetes API for Pod endpoints
- Falls back to static nodes if serviceDiscovery.enabled is false
- Added documentation in values.yaml explaining the feature

The RBAC permissions (services, endpoints watch) were already configured
in rbac-adc.yaml, so no additional permissions are needed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

helm/api7ee-demo-k8s/values.yaml

@@ -258,6 +258,11 @@ api7:
     key: ""
 
   # Service Discovery
+  # When enabled, API7 Gateway dynamically discovers backend Pods through Kubernetes API
+  # instead of using static upstream nodes. This provides:
+  # - Automatic scaling: New Pods are automatically added to the upstream pool
+  # - Health checks: Only healthy Pods receive traffic
+  # - Zero downtime: Automatic updates during deployments and rollouts
   serviceDiscovery:
     enabled: true # Use Kubernetes service discovery
     namespace: "" # Leave empty to use release namespace