feat: DTOs and interfaces

Models/DTO/MissionValidationResult.cs

@@ -0,0 +1,8 @@
+namespace GamificationService.Models.DTO;
+
+public class MissionValidationResult
+{
+    public bool IsValid { get; set; }
+    public string Message { get; set; } = string.Empty;
+    public int? SuggestedExperience { get; set; }
+}