Fix: re-implement `UserEndpoint` as static style which works best with minimal API, simplify the DI framework.
Fix: remove no needed service register in main program.
**Note: This implementation is not in minimalAPI way and not optimized, expected to be changed soon.**
Add: `UserService` and its interface `IUserService`.
Fix: `UserEndpoint` is now in instance class style with DI to work.
Fix: change main program to work with above design.
