feat: pydantic models

[ahnsv]

Resolve #10

Hi,
The test code gets messy to make the models (attr, pydantic) compatible.
If it feels right, I want to merge them first, refactor them and apply more validation criteria on model fields in later PRs.

[alexanderjordanbaker]

@ahnsv A few high level questions:

• Is your goal to have support for attr and pydantic both going forwards?
• Is the V1 vs V2 model approach meant to be temporary or permanent?

[ahnsv]

@alexanderjordanbaker
I think it would be better if we can replace it. In this PR, I added v2 for compatibility.
Do you think that I should make it replace it in this PR?