Add database schema classes within repo 

Use something like [dataclasses](https://docs.python.org/3/library/dataclasses.html) and/or [Pydantic](https://github.com/pydantic/pydantic)

Since repo is coupled with the data model being used, it'll be good to have the schema documented + accessible in a way that is standardized in the code + Git-accessible

Could possibly make constructing the BigQuery queries easier + more generic too