fix: convert field value to String instead of casting

sometimes, a silly vendor will make their pass field
a number in JSON instead of a string;
the iOS Wallet app displays it just fine though

lib/pkpass/models/pass_structure_dictionary.dart

@@ -121,7 +121,7 @@ class DictionaryField {
   factory DictionaryField.fromJson(Map<String, Object?> json) =>
       DictionaryField(
         key: json['key'] as String,
-        value: DictionaryValue.parse(json['value'] as String),
+        value: DictionaryValue.parse(json['value'].toString()),
         attributedValue: json['attributedValue'] == null
             ? null
             : DictionaryValue.parse(json['attributedValue'] as String),