Implement endpoints with Services Dependency Injection.

**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.

OptixServe.Core/Services/UserService.cs

@@ -0,0 +1,25 @@
+using OptixServe.Core.Models;
+
+namespace OptixServe.Core.Services;
+
+public interface IUserService
+{
+    IEnumerable<User> GetUsers();
+    User? GetUserById(string Id);
+}
+
+public class UserService : IUserService
+{
+    public User? GetUserById(string Id)
+    {
+        throw new NotImplementedException();
+    }
+
+    public IEnumerable<User> GetUsers()
+    {
+        return [
+            new() { Id = "1234", UserName = "xxx", Password = "pass1" },
+            new() { Id = "5678", UserName = "yyy", Password = "pass2" }
+        ];
+    }
+}