[flagd] implement `retryGracePeriod` param:

[toddbaert]

• defaults to 5
• when provider disconnects, if disconnected time <= retryGracePeriod, provider emits STALE
• when disconnected time > retryGracePeriod provider is emits ERROR

See https://flagd.dev/reference/specifications/providers/#flagd-provider-lifecycle for full details and spec.