feat: add AWS_IAM SASL protocol

[kjdelisle]

Uses the unique UserId of an IAM identity, the Access Key ID,
Secret Access Key and optional Session Token to authorize with a
Kafka broker.

The Kafka broker must also have the SASL/AWS login module
from kafka-auth-aws-iam in order to make use of this auth mode.

[tulios]

This is really cool @kjdelisle

Can you add a snippet on how the configuration looks like in the docs?
https://github.com/tulios/kafkajs/blob/master/docs/Configuration.md#-sasl

Something like:

sasl: {
  mechanism: 'aws',
  authorizationIdentity: 'fooMacFoo', // IAM user
  accessKeyId: 'AKIAIOSFODNN7EXAMPLE',
  secretAccessKey: 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY'
}

and maybe a link to https://docs.aws.amazon.com/general/latest/gr/aws-sec-cred-types.html#iam-user-name-and-password as people usually get confused about where to get these things.

[tulios]

Can you also keep this as 1.8.0? We will bump the version once we release 1.9

[kjdelisle]

@tulios Sorry that took so long! I was writing up some additional documentation for an edge-case where users would need to trim the caller-specified-role-id from a returned aws:userid, when I realized it would be better if the broker plugin simply handled that case for them.

Once I fixed it, tested it and released it, I came back to rewrite the rest of the documentation here (and hopefully make it a lot easier for users to deal with!)