feat: evaluate the user in the sdk instead of the backend

[Alessandro Yuichi Okimoto]

The current architecture when the get variation interface is called. It requests the evaluation directly from the server.
This approach is more straightforward but may introduce latency. Also, this could lead to infra-cost issues because it could evaluate the same user many times in a short time, even if the flag didn't change.

We should consider implementing the evaluation logic in the SDK to implement a cache similar to the client SDK.
This would improve the latency issues and reduce the requests to the server drastically, reducing the infra-cost.