[flagd-provider] Implement standard caching, with LRU cache

[toddbaert]

Implement standard flagd caching. Use a LRU cache implementation such as caffeine or similar. The cache requires a persistent connection to the flagd event stream. The stream should reconnect automatically if broken, and purge entries if change events are received or the connection fails.

See JS or Java implementation for reference.

Depends on: #31

[bacherfl]

@toddbaert I would like to work on this one :)

[toddbaert]

@toddbaert I would like to work on this one :)

assigned!