add a UserManager to simplify user creation, improve response validation in ServiceBase, allow Section component to support parameter changes

Oqtane.Server/Infrastructure/Interfaces/IUserManager.cs

@@ -0,0 +1,10 @@
+using System.Threading.Tasks;
+using Oqtane.Models;
+
+namespace Oqtane.Infrastructure
+{
+    public interface IUserManager
+    {
+        Task<User> AddUser(User user);
+    }
+}