Implemented user deletion service and IUserDeleteHandler for plugins to hook into

Moonlight.ApiServer/Interfaces/IUserDeleteHandler.cs

@@ -0,0 +1,10 @@
+using Moonlight.ApiServer.Database.Entities;
+using Moonlight.ApiServer.Models;
+
+namespace Moonlight.ApiServer.Interfaces;
+
+public interface IUserDeleteHandler
+{
+    public Task<UserDeleteValidationResult> Validate(User user);
+    public Task Delete(User user, bool force);
+}