[FA-10] Adds user service and authentication service

FictionArchive.Service.UserService/GraphQL/Mutation.cs

@@ -0,0 +1,13 @@
+using FictionArchive.Service.UserService.Models.Database;
+using FictionArchive.Service.UserService.Services;
+
+namespace FictionArchive.Service.UserService.GraphQL;
+
+public class Mutation
+{
+    public async Task<User> RegisterUser(string username, string email, string oAuthProviderId,
+        string? inviterOAuthProviderId, UserManagementService userManagementService)
+    {
+        return await userManagementService.RegisterUser(username, email, oAuthProviderId, inviterOAuthProviderId);
+    }
+}