Add support for passing device_id to the PostHog /flags endpoint.
This allows clients to identify devices separately from users when
evaluating feature flags.
- Add DeviceId field to FlagsRequestData (omitted when nil)
- Add DeviceId to FeatureFlagPayload and FeatureFlagPayloadNoKey
- Thread device_id through all flag evaluation paths
- Add tests verifying device_id is passed in request payload