Implement Vault Event Notification System for Kvv2 Hot Reload

Users would like to be able to reload Kvv2 but are currently not capable because the secret engine does not implement a ttl. Without a TTL the kvv2 engine cannot utilize the LeaseListener.

I propose we leverage Vaults Event Notification System (enterprise only) to listen for changes to kvv2 secrets and enable the client to hot reload. The mechanism works via websockets which can be implemented via via springs built in websockets functionality. The specific events would be kv-v2/data-{patch,write}.

Project Specific Questions:

• What project requirements are there for implementing a Enterprise only feature (any flags I need to set, etc?)

Open Questions:

• What changes need, if any, to bubble up to spring config vault (i think none)?
• Design for implementing filtering where applications may need to subscribe to many secrets (details link)
• Is it possible for users who use spring config vault's kvv2 implementation to benefit from this function (i dont think so but worth exploring)?

Start by reviewing the LeaseListener reference and Vault Event Notification System documentation, focusing on the kv-v2/data-patch and kv-v2/data-write events and websocket approach described here. Resolve the enterprise-only requirements, filtering design, and Spring Cloud Config Vault compatibility before implementation; the issue currently names no files or tests and does not define a settled done condition.