Support AWS MSK IAM Auth

[ijrsvt]

• Adds support for AWS IAM Auth when connecting to MSK clusters.
• Uses general code structure from https://github.com/segmentio/kafka-go
• Closes #87

[ijrsvt]

@everesio I can't seem to assign reviewers for this--can you help?

[HarshadRanganathan]

@everesio Can you please help in reviewing and merging this PR as it will help many leveraging the proxy in AWS to adopt secure practices - thanks

[everesio]

@ijrsvt Thank you for you contribution