Allow extension type as Map keys if it extends one of primitive json types or has toJson() method

[AlexeyKatsuro]

Codegenerator logs error:

Could not generate fromJson code for map because of type ItemId.
Map keys must be one of: Object, dynamic, enum, String, BigInt, DateTime, int, Uri.

@JsonSerializable(explicitToJson: true)
class Item {
  Item({required this.id, required this.map});

  final ItemId id;
  
  // Problem here
  final Map<ItemId, ItemId> map; 

  factory Item.fromJson(Map<String, dynamic> json) => _$ItemFromJson(json);

  Map<String, dynamic> toJson() => _$ItemToJson(this);
}

extension type const ItemId(String id) {
  factory ItemId.fromJson(String id) {
    // Here could be some logic to parse the id
    return ItemId(id);
  }

  String toJson() {
    // Here could be some logic to convert the id to a string
    return id;
  }
}

Expected generated code

Item _$ItemFromJson(Map<String, dynamic> json) => Item(
      id: ItemId.fromJson(json['id'] as String),
      map: (json['map'] as Map<String, dynamic>).map(
        (k, e) => MapEntry(ItemId.fromJson(k as String), ItemId.fromJson(e as String)),
      ),
    );

Map<String, dynamic> _$ItemToJson(Item instance) => <String, dynamic>{
      'id': instance.id.toJson(),
      'map': instance.map.map((k, e) => MapEntry(k.toJson(), e.toJson())),
    };

Also, the case with explicitToJson: false could be tricky because in the ItemId class, toJson is just an extension method, and jsonEncode will simply unbox the String id value. This behavior is unexpected if the extension type has a toJson method.