When accessing the bootstrap brokers of a cluster configured with property

clientAuthentication: msk.ClientAuthentication.sasl({ iam: true })

One can access the bootstrap brokers from the command line:

aws kafka get-bootstrap-brokers --cluster-arn <ClusterArn>

This will return an object:

{
    "BootstrapBrokerStringSaslIam": "..."
}

This PR adds the ability to access the bootstrap brokers directly as a property of the Kafka cluster. This can now be
done via the cluster.bootstrapBrokersSaslIam property.

Fixes #18355

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