Support IAM auth when using a managed PostgreSQL from AWS or others

### Description Existing database setup seems to only suggest username/password auth for Postgres https://airflow.apache.org/docs/apache-airflow/stable/howto/set-up-database.html#setting-up-a-postgresql-database. It would be great to support IAM auth as well. ### Use case/motivation Modern security practices include moving away from username/password auth, e.g. to avoid stale credentials and potential credentials leaks. Some companies may not even support connecting with username/password anymore. ### Related issues https://github.com/apache/airflow/issues/31759 ### Are you willing to submit a PR? - [ ] Yes I am willing to submit a PR! ### Code of Conduct - [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)