[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.

Start by locating the repository's attr-based models and reviewing how they are used across the App Store Server API library. Compare the migration scope and tradeoffs for dataclass or pydantic, then establish which model behavior and tests must remain unchanged before deciding what done means.

Written by the indexing model from the issue text.