Add support for AWS_MSK_IAM SASL mechanism to racecar

AWS recently [introduced](https://docs.aws.amazon.com/msk/latest/developerguide/iam-access-control.html) a SASL mechanism for AWS_[MSK](https://aws.amazon.com/msk/)_IAM that allows Kafka clients to handle authentication and authorization with MSK clusters through [AWS IAM](https://aws.amazon.com/iam/).

What would it take to support it? 