Generic single-database configuration with an async dbapi.

## To generate new migrations: 
run from danswer/backend:
`alembic revision --autogenerate -m <DESCRIPTION_OF_MIGRATION>`

More info can be found here: https://alembic.sqlalchemy.org/en/latest/autogenerate.html

## Running migrations

To run all un-applied migrations:
`alembic upgrade head`

To undo migrations:
`alembic downgrade -X` 
where X is the number of migrations you want to undo from the current state