Add IAM Auth support

[bvalente]

Related to prometheus upstream PR prometheus#983 and issue prometheus#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

[it-percona-cla]

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