Add IAM Auth support

[bvalente]

Addressing #749

We're adding the option to authenticate to AWS RDS with IAM, as described in this guide: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.IAMDBAuth.Connecting.Go.html

To test, I used the following config file (with redacted values):

[client]
user=my-user
host=mydatabase.eu-central-1.rds.amazonaws.com
port=3306
aws-iam-auth=true
aws-region=eu-central-1
enable-cleartext-plugin=true
ssl-ca=global-bundle.pem

@SuperQ 🙂