Add support for IAM database authentication to Amazon RDS

Is your feature request related to a problem? Please describe. Passwords are bad. Identity-based authentication is good. If the cube postgres/maria/mysql drivers supported IAM authentication that would be nice.

Describe the solution you'd like Just add some env variable like CUBEJS_DS_POSTGRES_DB_USE_AWS_IAM=true and it magically works.

Describe alternatives you've considered Stick with using passwords and have to stuff around with rotation and distribution of credentials etc.

Additional context RDS IAM auth docs