Support for providing a callable for passwords

[HLFrye]

We would like to use IAM roles for authentication with our AWS RDS hosted Postgres instance. This AWS functionality is described here. This feature allows an application to request a temporary password using AWS credentials. The temporary password grants access to the database, but the password expires after 15 minutes.

In order to use this feature along with a connection pool, we need to be able to pass a callable that returns a string as a password, rather than a string itself. This callable will need to be called each time the connection pool needs to create a new connection.