Add APIKey authentication

Helpers/ApiKeyValidator.cs

@@ -0,0 +1,20 @@
+namespace TodoApi.Helpers
+{
+    public interface IApiKeyValidator
+    {
+        bool Validate(string? apiKey);
+    }
+
+    public class ApiKeyValidator(List<string>? apiKeys) : IApiKeyValidator
+    {
+        private readonly List<string>? _apiKeys = apiKeys;
+
+        public bool Validate(string? apiKey)
+        {
+            if (_apiKeys == null) return false;
+
+            // Verify the provided apiKey is in our configuration
+            return _apiKeys.Contains(apiKey!.ToLower());
+        }
+    }
+}