[Suggestion] use dataclass or pydantic instead of attr for models.

Hi, just bumped into this repo while I was browsing apple's changed receipt validation policy.
Though it seems like this repo is its early stage, I think it would be more straightforward for potential users to use dataclass or pydantic.

首先定位 repository 中基于 attr 的模型，并审查它们在整个 App Store Server API 库中的使用方式。比较迁移范围以及采用 dataclass 或 pydantic 的权衡，然后确定哪些模型行为和测试必须保持不变，再决定什么才算完成。

由索引模型根据 Issue 内容生成。