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]

All committers have signed the CLA.

[bvalente]

Hello @JiriCtvrtka thank you for your review. Is there a schedule or something else needed to merge the PR? 😊

[bvalente]

Hi again @JiriCtvrtka ! Would it be posible to go ahead with merging this PR? Please let me know if you need any more information or feedback.

[JiriCtvrtka]

@bvalente Yeah, sorry, I completely missed this one. I’m going to add some tests for the newly added fields, then ask QA whether we can proceed or if it needs to be tested manually.

[JiriCtvrtka]

@bvalente I dont have permissions to push into your branch. I added test there: 9b6b5b9. Feel free to cherrypick this commit.

[bvalente]

@JiriCtvrtka Thank you for the test files. I've cherry-picked your commit into my branch