Add reason enum to decision engine rejections

Co-authored-by: Mark McDowall <mark@mcdowall.ca>

src/NzbDrone.Core/DecisionEngine/DownloadSpecDecision.cs

@@ -0,0 +1,34 @@
+namespace NzbDrone.Core.DecisionEngine
+{
+    public class DownloadSpecDecision
+    {
+        public bool Accepted { get; private set; }
+        public DownloadRejectionReason Reason { get; set; }
+        public string Message { get; private set; }
+
+        private static readonly DownloadSpecDecision AcceptDownloadSpecDecision = new () { Accepted = true };
+        private DownloadSpecDecision()
+        {
+        }
+
+        public static DownloadSpecDecision Accept()
+        {
+            return AcceptDownloadSpecDecision;
+        }
+
+        public static DownloadSpecDecision Reject(DownloadRejectionReason reason, string message, params object[] args)
+        {
+            return Reject(reason, string.Format(message, args));
+        }
+
+        public static DownloadSpecDecision Reject(DownloadRejectionReason reason, string message)
+        {
+            return new DownloadSpecDecision
+            {
+                Accepted = false,
+                Reason = reason,
+                Message = message
+            };
+        }
+    }
+}