Use `@JsonValue` annotation for Enum value when value is specified

[Shinya Kato]

1. What could be improved

Like:

{
  "enums": {
    "product_type": [
      {
        "name": "sweet",
        "value": 0
      }
    ]
  }
}

import 'package:json_annotation/json_annotation.dart';

enum ProductType {
    @JsonValue(0)
    sweet(0);

  final int value;

  const ProductType(this.value);
}

2. Why should this be improved

3. Any risks?

4. More information