Move python code into separate files

[neel-astro]

• [x] I have checked that a similar feature request does not already exist.

✍️ Is your feature request related to a problem? Please describe. The idea is to maintain python code in a separate python file instead of the current way of maintaining it as a static Golang string

Current files:

• https://github.com/astronomer/astro-cli/blob/main/airflow/include/dagintegritytest.go
• https://github.com/astronomer/astro-cli/blob/main/airflow/include/dagintegritytestdefault.go
• https://github.com/astronomer/astro-cli/blob/main/airflow/include/advancedexampledag.go
• https://github.com/astronomer/astro-cli/blob/main/airflow/include/basicexampledag.go

This will allow us to maintain that logic as python code and apply python linting and black