Added endpoint for registering a user locally after successful logon from Auth0

YABA.Service/UserService.cs

@@ -0,0 +1,42 @@
+using System.Linq;
+using YABA.Data.Context;
+using YABA.Models;
+using YABA.Service.DTO;
+using YABA.Service.Interfaces;
+
+namespace YABA.Service
+{
+    public class UserService : IUserService
+    {
+        private readonly YABAReadOnlyContext _roContext;
+        private readonly YABAReadWriteContext _context;
+
+        public UserService (YABAReadOnlyContext roContext, YABAReadWriteContext context)
+        {
+            _roContext = roContext;
+            _context = context;
+        }
+
+        public bool IsUserRegistered(string authProviderId)
+        {
+            return _roContext.Users.Any(x => x.Auth0Id == authProviderId);
+        }
+
+        public UserDTO RegisterUser(string authProviderId)
+        {
+            if(IsUserRegistered(authProviderId))
+            {
+                var user = _roContext.Users.FirstOrDefault(x => x.Auth0Id == authProviderId);
+                return new UserDTO(user);
+            }
+
+            var userToRegister = new User
+            {
+                Auth0Id = authProviderId
+            };
+
+            var registedUser = _context.Users.Add(userToRegister);
+            return _context.SaveChanges() > 0 ? new UserDTO(registedUser.Entity) : null;
+        }
+    }
+}