feat(extension): add custom URL handler support for extensions

- Add URLHandlerConfig to extension manifest (Go)
- Add HandleURL method to extension providers (Go)
- Add export functions for URL handling (Go)
- Add URLHandler class to extension_provider.dart (Flutter)
- Add platform bridge methods for URL handling (Flutter)
- Update fetchFromUrl to check extension URL handlers first
- Add Android/iOS native handlers for extension URL routing
- Update CHANGELOG with new feature

ios/Runner/AppDelegate.swift

@@ -484,6 +484,25 @@ import Gobackend  // Import Go framework
             if let error = error { throw error }
             return response
             
+        // Extension URL Handler API
+        case "handleURLWithExtension":
+            let args = call.arguments as! [String: Any]
+            let url = args["url"] as! String
+            let response = GobackendHandleURLWithExtensionJSON(url, &error)
+            if let error = error { throw error }
+            return response
+            
+        case "findURLHandler":
+            let args = call.arguments as! [String: Any]
+            let url = args["url"] as! String
+            let response = GobackendFindURLHandlerJSON(url)
+            return response
+            
+        case "getURLHandlers":
+            let response = GobackendGetURLHandlersJSON(&error)
+            if let error = error { throw error }
+            return response
+            
         // Extension Post-Processing API
         case "runPostProcessing":
             let args = call.arguments as! [String: Any]