Migrate to Pydantic v2 #28

Currently, we use Pydantic v1 - this prevents users from using the google-genai library, which relies on Pydantic v2.

There is a migration guide that describes the process.

The new version should be v1.0.0 to not break compatibility with users who rely on Pydantic v1 version that will still live in v0.X.X.