Implement flagd RPC caching as per spec. At a high level, "STATIC" flag values can be cached in an LRU cache

• identify adequate LRU cache lib or implement one
• implement flagd event stream in initialize
  • stream must reconnect with connection failure (some implementations include an API for listening for state changes in the underlying connection, this can be very useful for this feature)
• shutdown event stream in shutdown.
• use cache as per spec
• tests