feat: 956 - added product owner fields

New file:
* `owner_field.dart`: Helper class to compute the Product.ownerFields tags.

Impacted files:
* `api_get_product_test.dart`: added a test for owner fields
* `openfoodfacts.dart`: added new class `OwnerField`
* `product.dart`: added field `ownerFields` and method `getOwnerFieldTimestamp`
* `product.g.dart`: generated
* `product_fields.dart`: refactored around localized product fields

lib/openfoodfacts.dart

@@ -32,6 +32,7 @@ export 'src/model/old_product_result.dart';
 export 'src/model/ordered_nutrient.dart';
 export 'src/model/ordered_nutrients.dart';
 export 'src/model/origins_of_ingredients.dart';
+export 'src/model/owner_field.dart';
 export 'src/model/packaging.dart';
 export 'src/model/parameter/allergens_parameter.dart';
 export 'src/model/parameter/barcode_parameter.dart';

lib/src/model/owner_field.dart

@@ -0,0 +1,25 @@
+import '../utils/language_helper.dart';
+import '../utils/product_fields.dart';
+import 'nutrient.dart';
+import 'off_tagged.dart';
+
+/// Helper class to compute the Product.ownerFields tags.
+class OwnerField implements OffTagged {
+  const OwnerField.raw(this.offTag);
+
+  OwnerField.nutrient(final Nutrient nutrient) : this.raw(nutrient.offTag);
+
+  factory OwnerField.productField(
+    final ProductField productField,
+    final OpenFoodFactsLanguage language,
+  ) {
+    final ProductField? inLanguages = productField.inLanguages;
+    if (inLanguages == null) {
+      return OwnerField.raw(productField.offTag);
+    }
+    return OwnerField.raw('${inLanguages.offTag}${language.offTag}');
+  }
+
+  @override
+  final String offTag;
+}

lib/src/model/product.dart

@@ -10,6 +10,7 @@ import 'ingredients_analysis_tags.dart';
 import 'knowledge_panels.dart';
 import 'nutrient_levels.dart';
 import 'nutriments.dart';
+import 'owner_field.dart';
 import 'product_image.dart';
 import 'product_packaging.dart';
 import '../interface/json_object.dart';
@@ -562,10 +563,19 @@ class Product extends JsonObject {
   )
   bool? obsolete;
 
+  /// Timestamps of owner_fields, in seconds.
+  ///
+  /// Typically used to say "Was that field set by the owner?".
+  /// Read-only.
+  /// See also [getOwnerFieldTimestamp].
+  @JsonKey(name: 'owner_fields', includeIfNull: false)
+  Map<String, int>? ownerFields;
+
   /// Expiration date / best before. Just a string, no format control.
   @JsonKey(name: 'expiration_date', includeIfNull: false)
   String? expirationDate;
 
+  // TODO(monsieurtanuki): remove all the "this" constructor fields, except maybe "barcode"
   Product(
       {this.barcode,
       this.productName,
@@ -766,7 +776,7 @@ class Product extends JsonObject {
           result.imagesFreshnessInLanguages![language] = values;
           return;
         default:
-          if (fieldsInLanguages.contains(productField)) {
+          if (productField.isInLanguages) {
             throw Exception('Unhandled in-languages case for $productField');
           }
       }
@@ -800,7 +810,10 @@ class Product extends JsonObject {
       // We store those values in a more structured maps like
       // [productNameInLanguages].
 
-      ProductField? productField = extractProductField(key, fieldsAllLanguages);
+      ProductField? productField = extractProductField(
+        key,
+        ProductField.getAllLanguagesList(),
+      );
       if (productField != null) {
         final Map<OpenFoodFactsLanguage, String>? localized =
             _getLocalizedStrings(json[key]);
@@ -813,7 +826,10 @@ class Product extends JsonObject {
         continue;
       }
 
-      productField = extractProductField(key, fieldsInLanguages);
+      productField = extractProductField(
+        key,
+        ProductField.getInLanguagesList(),
+      );
       if (productField != null) {
         final OpenFoodFactsLanguage language =
             _langFrom(key, productField.offTag);
@@ -1073,4 +1089,12 @@ class Product extends JsonObject {
     // quantity: put non breaking spaces between numbers and units
     return '$productNameBrand$separator${quantity!.replaceAll(' ', '\u{00A0}')}';
   }
+
+  /// Returns the timestamp (in seconds) of the owner field.
+  ///
+  /// Typically used to check if the field value was set by the product owner
+  /// (if the timestamp is not null), which means that the user cannot set a
+  /// value.
+  int? getOwnerFieldTimestamp(final OwnerField ownerField) =>
+      ownerFields?[ownerField.offTag];
 }