Refactor to support new features and code maintainability

[jlaneve]

This is a pretty large refactor of how we were doing parsing and generation to support a few goals:

• General code maintainability and extensibility
• Subclass Airflow's DAG and TaskGroup model instead of returning them
• Remove pydantic dependency in favor of dataclasses (closes Remove pydantic dependency #74)
• Add different testing types (after_each, after_all, none) (closes Configure testing behavior #62)
• Add tags support (from code and from yml config) (partially addresses Create an Airflow DAG/Task Group from a dbt selector argument #67)
• Add docs for testing types and tags

[chrishronek]

Tested - LGTM!