Upgrade core VAULT for multiple KEYS

Generate sub keys like KEY:HEALTHZ:SOMETHING or global key shared by KEY:ENGINE:SOMEGLOBALKEY

Healthz shutdown security:

• Extend Vault to support multiple explicitly authorized keys per layer/provider.
• Move the shutdown secret to Vault as KEY:HEALTHZ:SHUTDOWN.
• Authenticate /shutdown through an Authorization Bearer header.
• Avoid secrets in query strings.
• Do not mount the shutdown endpoint when its required key is missing.
• Emit an explicit configurable warning when shutdown authentication is unavailable.