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/secret-api7.yaml

@@ -1,4 +1,4 @@
-{{- if .Values.api7.enabled }}
+{{- if and .Values.api7.enabled (not .Values.api7.gateway.existingSecret) }}
 apiVersion: v1
 kind: Secret
 metadata: