Implemented user deletion service and IUserDeleteHandler for plugins to hook into

Moonlight.ApiServer/Models/UserDeleteValidateResult.cs

@@ -0,0 +1,27 @@
+namespace Moonlight.ApiServer.Models;
+
+public class UserDeleteValidationResult
+{
+    public bool IsAllowed { get; set; }
+    public string Reason { get; set; }
+
+    public static UserDeleteValidationResult Allow()
+    {
+        return new UserDeleteValidationResult()
+        {
+            IsAllowed = true
+        };
+    }
+
+    public static UserDeleteValidationResult Deny()
+        => Deny("No reason provided");
+
+    public static UserDeleteValidationResult Deny(string reason)
+    {
+        return new UserDeleteValidationResult()
+        {
+            IsAllowed = false,
+            Reason = reason
+        };
+    }
+}