Major rewrite of the library, updating it to work with Pydantic v2.
It is heavily inspired on the new [Pydantic Settings](https://github.com/pydantic/pydantic-settings) package, as previous version were from Pydantic's pydantic.env_settings [subpackage](https://github.com/pydantic/pydantic/blob/v1.10.12/pydantic/env_settings.py).