Support non-public schema for PostgreSQL 15+

### Description For PostgreSQL setup, the doc says: -- PostgreSQL 15 requires additional privileges: GRANT ALL ON SCHEMA public TO airflow_user; For security-concerned customers that do not want to use the public schema, it would be nice if the schema to use could be configurable. ### Use case/motivation For security sake, use a schema other than public. ### Related issues _No response_ ### Are you willing to submit a PR? - [X] 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)