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](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.IAMDBAuth.html)