Add `airflow db autogenerate` command for generating migration files

[ephraimbuddy]

This PR customizes the alembic autogenerate command. The DB is reset using the migration files helping to determine new additions in the ORM accurately

[potiuk]

Should we add description about it to https://github.com/apache/airflow/blob/main/contributing-docs/13_metadata_database_updates.rst ? I think that one needs a bit more love and describe the process of adding migration scripts in a bit more detail for anyone who wants to do it ?

[uranusjr]

Should this be an airflow command? I feel the end users probably never need this. It would fit better for this to be a breeze command instead, or just a script in the repo.

[potiuk]

Should this be an airflow command? I feel the end users probably never need this. It would fit better for this to be a breeze command instead, or just a script in the repo.

Yep. Breeze command would seem most appropriate.