New: Newznab/Torznab categories dropdown with indexer provided category names

Signed-off-by: Robin Dadswell <robin@dadswell.email>

src/NzbDrone.Core/Indexers/Newznab/Newznab.cs

@@ -138,5 +138,31 @@ namespace NzbDrone.Core.Indexers.Newznab
                 return new ValidationFailure(string.Empty, "Unable to connect to indexer, check the log for more details");
             }
         }
+
+        public override object RequestAction(string action, IDictionary<string, string> query)
+        {
+            if (action == "newznabCategories")
+            {
+                List<NewznabCategory> categories = null;
+                try
+                {
+                    if (Settings.BaseUrl.IsNotNullOrWhiteSpace() && Settings.ApiPath.IsNotNullOrWhiteSpace())
+                    {
+                        categories = _capabilitiesProvider.GetCapabilities(Settings).Categories;
+                    }
+                }
+                catch
+                {
+                    // Use default categories
+                }
+
+                return new
+                {
+                    options = NewznabCategoryFieldOptionsConverter.GetFieldSelectOptions(categories)
+                };
+            }
+
+            return base.RequestAction(action, query);
+        }
     }
 }