Add support for existing Secrets and External Secrets Operator

Enhanced secret management for API7 Gateway credentials with support
for existing Secrets and External Secrets Operator integration.

Changes:

1. Secret Configuration:
   - Added api7.gateway.existingSecret parameter for using existing secrets
   - Added api7.gateway.existingSecretKeys for custom key names
   - Modified secret-api7.yaml to only create secret if existingSecret is empty
   - Updated job-adc-sync.yaml to reference configurable secret name

2. Values.yaml Documentation:
   - Added comprehensive documentation for secret configuration options
   - Documented two approaches: inline config (dev) vs existing secret (prod)
   - Added example kubectl command for creating secrets manually
   - Included instructions for obtaining admin key from API7 EE

3. External Secrets Support:
   - Created externalsecret-api7.yaml.example with complete examples
   - Included examples for AWS Secrets Manager and HashiCorp Vault
   - Documented SecretStore configuration patterns

4. Documentation:
   - Created SECRET-MANAGEMENT.md comprehensive guide
   - Covered all secret management options (inline, manual, external)
   - Added security best practices and troubleshooting guide
   - Included examples for External Secrets Operator setup

Benefits:
- Improved security: Secrets not stored in values.yaml
- Flexibility: Support for any secret management tool
- Production-ready: Works with External Secrets Operator
- Better practices: Clear separation of config vs secrets

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

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

helm/api7ee-demo-k8s/templates/job-adc-sync.yaml

@@ -47,18 +47,18 @@ spec:
             - name: API7_ADMIN_URL
               valueFrom:
                 secretKeyRef:
-                  name: {{ include "api7ee.fullname" . }}-api7-admin
-                  key: admin-url
+                  name: {{ .Values.api7.gateway.existingSecret | default (printf "%s-api7-admin" (include "api7ee.fullname" .)) }}
+                  key: {{ .Values.api7.gateway.existingSecretKeys.adminUrl | default "admin-url" }}
             - name: API7_ADMIN_KEY
               valueFrom:
                 secretKeyRef:
-                  name: {{ include "api7ee.fullname" . }}-api7-admin
-                  key: admin-key
+                  name: {{ .Values.api7.gateway.existingSecret | default (printf "%s-api7-admin" (include "api7ee.fullname" .)) }}
+                  key: {{ .Values.api7.gateway.existingSecretKeys.adminKey | default "admin-key" }}
             - name: API7_GATEWAY_GROUP
               valueFrom:
                 secretKeyRef:
-                  name: {{ include "api7ee.fullname" . }}-api7-admin
-                  key: gateway-group
+                  name: {{ .Values.api7.gateway.existingSecret | default (printf "%s-api7-admin" (include "api7ee.fullname" .)) }}
+                  key: {{ .Values.api7.gateway.existingSecretKeys.group | default "gateway-group" }}
           volumeMounts:
             - name: adc-config
               mountPath: /config