Implement Apache Airflow provider's hook interface

[tomasfarias]

Not quite an interface as it's not defined in BaseHook, but most Apache Airflow providers define proper connection types for each of their hooks. This may be a chance to refactor the hooks module into individual hook classes for each dbt supported target type. Furthermore, this can be the chance to clean up the docstring documentation as it's missing a lot of information.

Perhaps we can apply to be integrated into Apache Airflow as an official provider? Currently, a dbt Cloud provider package exists, but nothing for "vanilla" dbt.